I wasn't a big fan of his in Smash 4, though I do think Punchout is a wonderful video game series. But I seemed to have liked him more in Ultimate, for whatever reasons that is.
I suppose Little Mac's problem is that all his special and smash moves seems to work about the same. He's just a boxer, so they can't get too creative. He does seem to have the power when he needs it, most of the time.
His recovery moves on the other hand, you need to be careful. If you don't plan on your return, you can easily fall to your doom.
His final smash turns him into a super-version of himself. This Hulk-Little Mac will cause a lot of pain to the fool caught in his way. But it is often too easy to miss with this one.
When Kirby takes his power, he gets a super-punch. While powerful, enemies can see it coming a mile away. So Kirby would be wise to go after another power, if possible.
But overall, I was a fan of Little Mac. Not enough for him to be in my top ten, but I had good luck with him online.