Umm, no. I had absolutely no problem defeating Little Mac players, because they actually killed themselves with their sideway special. When he has full meter just stay away from him... As Sonic, you can easily outrun him.

When he gets the meter full just run like a little bitch! Get away from him and wait until it goes off.

The fsmash is the attack he killed you in the end, which you charged directly into.  But on the subject of his KO punch, no you shouldn't run away from it for several minutes, but you also shouldn't whiff an attack an inch away from him.

Every character is OP if you're good enough.

Nah, you can abuse little mac easily. Little Mac is a monster when he controls the center, so just stay close to the edge and try to gimp him.

His approach options are pretty shit, so it shouldn't be hard to get him off stage, if you stay at the edge.

You need to be a bit more careful in your play. Your KO against him was good, getting Mac off the stage vertically but on your first stock you got carried away trying to hit Mac after he landed which is why you got hit and the second time you just ran into his FSmash.

Dodge dat KO punch dude :P

And you ran right smack into his last hit, you even had an easy oportunity to roll past him and he would have been exposed.