Close but no cigar. Metaknight was known to be OP to the point of being a broken character.
Only he wasn't. MK was highly debated from the moment of his top tier placement. Had several debatable evens(which at worst were 6/4s), including a character that could 0-death from a grab, and a character with an infinite on flat stages. Suffered from grab releases, died relatively early, and host of other weaknesses that were exploitable by players of equal skill. There was even a time when some players attempted to play MK and dominate to demonstrate how broken he was, only to get bopped and get similar tourney placements.
The biggest contributors to the "MK is broken/OP" schill were 1. A new generation of players that were unaccustomed to competitive play(basically casuals trying to transfer over) and the over centralization blamed on MK due to his evens being higher skill caps(This is why Melee Fox and 64 Pikachu ******** never reaches a boil).
Little Mac is just a character with all the right tools to be the most easily accessible character in the game. His strengths and weaknesses dominate low-skill gameplay because they're so easy to use and the other person isn't good or smart enough to take advantage of his weaknesses.
And because of that, he attracts ALL of the worst players who pick him expecting to win, and then they end up being ******** and dragging Little Mac's name through the dirt. It's really a shame but in a way it's perfect.
You can literally replace "Little Mac" with any top tier character's name in any game and have that be accurate.
People are consider bad for maining Little Mac for the same reasons and by the same type of people that hated Pikachu, Fox, and MK. Because they can't win consistently enough, when it feels like Mac players don't put in half the effort. Only in Mac's case, the main complainers will be predominantly online players because online is playable now widening the casual pool. As per usual in actual tournaments and in matches of equal skill, he'll look better, be much worse in the hands of pros and still find losses against people that play better.