Check Alan out, she is amazing.
EDIT: I see you want groups instead of solo.
I still say try Alan out, you might end up liking her. smile
Yousei Teikoku as well. Their a rock band with a female lead singer.
wow half the post in this thread are like female soloists, male soloists aka not fitting the criteria lolrolleyes
ClariS +1 (high schoolers so might be considered cutesy but Clara has a really mature voice)
Perfume +1 if you like electropop
try AMOYAMO if you like Perfume
Tenjochiki - old kpop group but sang good japanese
DiVA - subunit of AKB48, but not cutesy at all, theme is mature
BENNIE K - j-urban unit, try MONOCHROME or WONDERLAND