"But can that girl even sing any good?" Depends on your tastes and standards. I don't think anyone is going to compare her voice to Barbara Streisand, Celine Dion, Adele, etc. For a current pop star, I think she sings reasonably well. In my opinion, she sings better than Demi Lovato or Ke$ha, but not as well as Christina Aguilera or P!nk. She's probably on par vocally with Taylor Swift -- passable but not great voices -- but at least Swift writes her own songs (Gomez does not). Most of Gomez's songs are not autotuned. Her success is based on her image and tween to teen fanbase, not on her vocal ability or her acting. That said, she has transitioned pretty well compared to say, Miley.
I would guess that I probably listen to more current female pop stars than anyone else on this board -- radio, mp3s, live concerts. I saw Taylor Swift in concert last weekend, Ellie Goulding earlier this year, and will see Cher Lloyd and Kelly Clarkson in the next month or so.
@Draugnar: you forgot the category of Disney star who can sing, act, AND dance -- Justin Timberlake.