So, I guess I could talk about my thoughts on Little Mac's match ups a little bit.
I think Little Mac has a lot of trouble against the following:
ZSS: She's got range and is easily the best character at spacing Mac out, as the stun on her gun often stops Mac's super armor. She can also combo for long enough to outlast his KO punch, she's got great grab range which is troublesome for Mac, and she's pretty good in the air.
Wario: The bike will often override Mac's armor and he's also great in the air. The bite is also a very effective move against Mac.
Yoshi: Yoshi's move that lets him lay little Mac as an egg will oftentimes kill him if used on the edge due to his terrible recovery. Also, Yoshi has projectiles that travel in an arc, meaning Mac can not always run right under them like he does other proectiles. But, he also has super armor on enough moves that it can be troublesome for Little Mac.
Dedede: I can't put my finger quiet on why he gives Mac trouble, but he just does. Maybe it's the range? He can also blow Mac off the side and then gimp him. It's probably the range and the gimping abilities.
Other Little Macs: Little Mac dittos are honestly really hard for Mac, and can also feel like cheap contests consisting of who rolls better, counters first, or is lucky enough to hit with their KO punch first. However, they're completely even matches, but they're hard to learn because you can't exploit some of Little Mac's cheaper tactics like you can when going against a different character.