I think there's a lot of waifus who are underrated and not a lot of people likes or knows about

Anthea and Concordia from Pokemon BW, their presence in the game is short af but I really loved their simple designs and story with N

My immediate answer is Melona and Menace from Queen's Blade. I never really understood why they didn't get big fan art explosions. Such underrated waifus.

Also Chifusa from Manyuu Hikenchou and Tomo from Seikon No Qwaser don't have as much fan art as they should.

Tamamo from Kanokon. She's a 9-tail kitsune MILF with massive tits and it saddens me she doesn't have more fanart

Izumi Curtis. Kick ass mentor character and a beautiful mother like woman

Judy from Cowboy Bebop, Kushana from Nausicaa, Deis/Bleu from the Breath of Fire games, and Glacia from Pokemon RSE come to mind. I would say Senjumaru from Bleach in the sense of not a lot of posts of her but I do remember folks popping off for her after only like 3 scenes in the anime so it's difficult to call her underrated exactly.

Bullet from Blazblue and King from AOF

I do like King. She's one of my favorite SNK characters to play

Emi from Fighting Vipers. Her play style and ass slap intro instantly won me over. Honestly I wish Fighting Vipers had another game in general...

I always liked Shani from Witcher 3 Hearts of Stone