Mama Tua is strong on the original N64 port, every other port: PS1, Dreamcast, PS2, maybe even GBA however she got nerfed.

What's the best fighter to use if not her then?

I've got 9 minutes and 11 seconds as Hillary Clinton so I guess it's her.

How do I unlock her and what's her best combo since Mama Tua's was the overhead punch spam?

Play championship mode and finish the game on Easy with any character.