I've heard people say Ike is OP, Pit is OP, and Kirby is OP. Some know that MK is banned, but some that know this don't understand why.
If you teach your friend group to airdodge, shield, spotdodge, and grab, rock won't be OP, because they can avoid rock and punish it.
If you want them to unban Kirby, just ask one of them to play you and play your best character, doing your best to destroy them. If they all see you easily wreck Kirby (and tell them you won't play Kirby), they'll probably unban him.
n00bs say Kirby's OP because Kirby has a powerful smash attack in fsmash that doesn't leave tons of vulnerability (yes it's punishable but you don't have forever, unlike if you powershield Ike fsmash or something). His rolling is also not bad (I don't know if it's good per se...) so rolling plus fast smash with high knockback scaling = dangerous. He has 5 jumps and can steal powers which many think is way too good (Pit arrow problems? steal them).
Pit is also (among n00bs) widely considered OP because he has arrows, his angel ring, and his shield reflects stuff, while his recovery goes forever (a lot of n00bs don't get the concept of gimping, which makes his recovery more or less the best, next to Snake and an MK that doesn't SD, and of course Kirby). His aerials are all not that slow for new players, and fair's knockback is okay. He also has a dash attack with decent knockback scaling. His fsmash does a lot of damage and is fast. His dsmash is fast, as is his usmash.
Ike is considered (among n00bs) OP as well because [as one n00b put it] "a ******** monkey could win at Smash by pressing A and choosing three letters - I, K, and E." His jab combo is really good (one of the few things n00bs get right, although they then overestimate its effectiveness), and since n00bs aren't necessarily good at aerials or smashes/tilts (unless they have cstick), Ike has a really good attack. Given the lack of shielding and dodging from n00bs, and Ike's wide attack area on fsmash and usmash, his attacks are hard to avoid for them, so he hits hard and in a large area. His recovery isn't great, but for n00bs who sort of know how to gimp, Ike is still mostly ungimpable.
I haven't run into many "Pika OP" people, but n00bs just say he has a "spammy" moveset, namely t-jolt and Thunder2, as mentioned above. n00bs also think his dsmash is way too good because they don't understand SDI.
I ran into one GnW and ROB OP n00b because cstick down and dtilt (and projectiles + side+B for ROB). I think this is the exception, not the norm though.
No real experience with TL OP n00bs, although dair and his fsmash are a lot of damage and hard to dodge/punish for n00bs.
[I used to be called a spammer quite frequently, but as I like to point out, if you could use one of your moves and get hit, or use a move and hit them, is it spamming if you use it? No? Okay, if you could use one of your moves and get hit, or use the same move you just used and hit them, is it spamming if you use it? No? Okay, Okay, if you could use one of your moves and get hit, or use the same move you just used and hit them, is it spamming if you use it? No? Okay, if you could use one of your moves and get hit, or use the same move you just used and hit them, is it spamming if you use it? No? Okay... (it repeats until they realize my point or set an arbitrary cap, at which I point out the arbitrary cap, and if it's not retracted I just laugh at them.)]
Most who don't get why MK was banned think his knockback is way too low (they don't get the idea of racking damage and stale moves), and they think he's easy to SD with (because most will Mach Tornado offstage or Drill Rush or Shuttle Loop at a bad time). They also think he's too light (though this never comes up with Kirby...).