Generally an oldish woman with excess body fat from having one too many kids. She generally doesn't appreciate kids singing that song, neither does her kid. Because quite frankly, and 40+ year old women who you wouldn't even want to see in a tank top, is not attractive. Unless you're into that kinda thing.
Stacey: Hi, my name's Stacey.
Person: Hey,like the song Stacey's Mom! Your mom's got it going on!
Stacey: No, not unless you like fat hags.