Assuming ‘non-mainstream’ means not popular, it seems like quite a few of my favorites fit.
Bangle, Cole, Graham, Soleil, Willow, Ike, Ursala, Stinky, Alice, Sydney, Clay, Dierdre, Antonio, Freckles, and Maggie are some of my (many) faves who I believe fall on the unpopular/ unknown end of the scale.
Tabby is an unusual case, because I would argue she is well known for being unpopular in a way similar to but not to the same extent as poor Rodney. I would say they would be in the mainstream as unpopular villagers. Mainstream, non-mainstream, or something in between, I do love Tabby so much.
Genji, Flurry, Apple, Ruby, Agnes, and Fauna are favorites I would consider ‘mainstream’/ popular