From my experience, Little Mac is not Op. He is not brawl Meta Knight and he is definitely not Pre Patch Sm4sh Luigi. For a character to be OP IMO, they need to have too many advantages to the point where fighting that character with most, if not all, of the cast is unreliable at best and outright disastrous at worst.
Mac's Ground Game, without any doubt, is one of the best, if not the best in the game. Top tier, no doubts about it. However, it takes A LOT of skill and experience to master it. Not only that, but Mac has been designed that in compensation for the Ground Game, His grab game and Safe moves game take a toll. All of his moves are unsafe and can be punished if the opponent has the right character and skill to do so. His Grab game is Mediocre at best since the grab itself is unreliable and even if you do land it, there are no guaranteed follow ups (Which usually relies on the opponent failing a tech) and none of the throws are kill throws except on Sudden death (Specifically B-Throw and U-Throw) This is of course without including his subpar air game and terrible recovery.
Another interesting part about Mac is that he is a gambler similar to the likes of Falcon. If you know what your doing, you can make something out of it.
In the end, if someone complains that your are being cheap or Mac is op. Pretty much do the same that a Rosalina Main would ask to that complainer: "Play the character and tell me how easy it is to win games against High Level players without dedicating time to this character."