So just for kicks (don't take this too seriously), here is a rough tier list of which characters (and emphasized by certain players that do certain things) are annoying / unfun to play against.
We play 1v1 and 2v2, team damage turned off, avoiding items and ultimate smash attacks. Note, the kid is really stinking good at this game... like destroys in local tournaments and fights some of the hardest guys online sometimes, winning with 1v1 low tier characters a lot. Me, I'm still a newb at this game, but been playing fighting games since they were invented, casually mostly.
As for the Unfun Tier List, it's my first attempt at this list - based on my low level playing and the kid's high level, so sort of a mix. I'll explain some thoughts on the ones that stand out.
LEGEND: Turtling/Turtler = Camping/Camper = too defensive = running away from the fight and trying to win by long range attacks or avoiding conflict.
NOTE: No offense intended towards people's favorite characters. This is just in this game, I don't typically dislike these characters outside of Smash Bros.
Top of the list here...
S+ Tier punks:
The pink blobs... I just don't like their look, apologies to fans. I can't tell what move they are doing... I mean their arms and legs don't look like much, so I never know if I got hit by a jab or a kick or a smash attack half the time, and when it hits hard I just die. They float around too much, irritating me like no other.
And at my level, Kirby specifically annoys me in 2v2 because he just floats around forever and ever, dragging out the game even when he's lost by a mile already. The pink cat blob has the sleep thing... and at my cruddy level, it gets me way too much. Or worse, I avoid it, then I try to get in for a punish and screw that up. Annoying annoying annoying all around.
Steve is obvious, too defensive/runs away. Crazy how infrequently you see him, it's almost like nobody likes using him (which is great, since fighting against him is just silly).
Yoshi ... I can't put my finger on it, but every time I play against a Yoshi I am left annoyed, win or lose. Maybe it's his annoying sounds (Sorry Yoshi, you're cool in the Mario Bros. games) or the way people play him, but it is pretty consistent for me to be annoyed. I think a lot of higher skilled guys play him at my low GSP rating and that's half the issue.
Pikachu - Again, nothing against him in other places, but in this game: annoying sounds, hard to hit b/c he's barely the size of an ant, and his attacks just are surprising brutal in a way that makes me grit my teeth.
Samus is a turtling piece of @#$% in this game. :) Really though, the keepaway game is just too frustrating for some of the cast. In higher level 1v1 play, if you are in certain distances and try to counter the projectile, Samus can react and punish you easily.
Pyra of Pyra/Mythra - I need her up here in S+ Tier b/c in 2v2, that flame sword is stupid. I mean, does it ever not hit me in 2v2? In 1v1, she's tough but not ultimately annoying compared to some. Sidenote: Mythra stupid b/c can basically never punish the side-B slashy slash stuff.
S Tier:
Sora... at low levels, just too annoying b/c you have to wait and wait and then try to punish after blocking the right thing. Makes me impatient and annoyed.
The floating ladies like Peach... in the S tier, they just get under my skin. Teleporting, running away, you name it. I've had fun once where I kept finding a counter to the Knight guy she summons... more of a "I hate this so much, I'm glad I beat you" feeling though.
Minmin - terrible. Like Dhalsim in SSF2T. Breaks the game for so many characters. I can't believe she has a way to jump, wiggle her arms out horizontally, and cover like a huge area safely for practically the entire jump.
Megaman/Pacman - runs away like crazy, too many projectiles. And Pacman never dies, I mean never dies... just slowly gets back up there somehow.
Dedede - more the player than the character. But his mega hard hits are a shocker to my low tier eyes. In 2v2, these guys just ooze slime. Super defensive projectiles when you least want to deal with them. Aggressive teabagging, etc. Apparently even in higher level play, the slimelords run this character.
Link - too many projectiles, runs away. His sword stays out forever on like down-air and stuff, and snuffs out every single thing... it's insane.
Mythra of Pyra/Mythra - Mythra is stupid b/c can basically never punish the side-B slashy slash stuff.
A Tier:
Kazuya - (He'd be S+ tier for the kid) - hits so hard, you will be surprised when you die. At higher levels of competition, his combos are very hard to escape and do tremendous damage even when you know almost all the tricks to escape them. Kudos to Riddles for explaining how to deal with Kazuya combos in his vids. We still can't do it, but at least there is hope. To be fair, the kid plays mostly the biggest heavies, so it's probably 3 times as hard to escape combos. [In reality, at my level he isn't an issue yet... nobody much can do the crazy combos that well.]
Mario/Luigi - Luigi's sound when he does his dash attack, makes me insane, Grrrr. Mario is just always played by guys about 20 times better than me, but somehow their GSP is the same as mine for Mario. WHY? And his forward air spike... the punks.
More overly keepaway turtly play by several in this tier.
King K.Rool - the 90% of players at my level in 2v2 try to act as annoying as possible. "They think they are funny, but they just aren't" is what the kid says. I do like using this character myself though.
Cpt. Falcon - The dude is only played by pros who somehow are at my GSP. That anti-air Knee... Gaaaah!
Diddy Kong - obnoxious fly.
Ness - PKfire is a nightmare in 2v2.
B Tier:
Not much to say other than... most ppl hate fighting good Sonics. At my level, the Sonics don't annoy me somehow so I can't complain.
Mr. Game & Watch - I use him, and so playing against him makes sense/no surprises mostly. Maybe his back-air is just too good.
The rest of the cast here... I don't have enough experience against to really give good opinions.
C Tier and D Tier:
These guys don't typically have any obnoxious play that I see at my level so, sure, I'll fight 'em.
Ken/Ryu - Now if it were Street Fighter, I'd put Ken and Ryu near the worst of fun to fight against. Somehow Smash Bros. makes their turtling 100 times less annoying than in Street Fighter.
Incineroar - I use him a lot. But playing against him, he's so scary in 2v2. I never know when I'm going to die.
Wario - he's funny, what can I say.
Pokemon Trainer - I just like the concept, cool.
NOTE: A good reference, I'm finding, is this one which is "which char.s are great in casuals, including the SuperMove aka Final Smash and items and junk": https://www.reddit.com/r/SmashBrosUltimate/comments/zkclhy/an_official_casual_tier_list_from_a_seasoned/
NOTE: References to make tier lists, etc. are:
https://tiermaker.com/categories/super-smash-bros-ultimate/tier-lists/
