Clay Men would be tough enemies to beat, but would be very slow.
Fobbies and Foppies are perfect just for punching bags. Also, they're very popular to Mother fans.
Various Pigmasks would be an obvious choice, just to bash them around until they squeal.
Giving some Bosses some love as mini-bosses would be cool, too.
Starmen and its variations would be perfect. They're probably the most popular enemy in the entire Mother franchise.
Mother has perhaps the craziest, fun, and wildest enemies in video games to date. I think that if Nintendo can't release the games in the US due to copyright, they should at least give a little extra content in Smash Bros. After all, it's been 16 years since we last saw a Mother game in the US.