Learn English – A term to describe “thinking something bad is good”

A wolf in sheep's clothing is someone who tries to trick you into trusting them by presenting themselves as unthreatening, then unexpectedly attacks you.

A poser is someone who pretends to have a "better" social/intellectual/economic position than they really do. @Jay's example is slightly odd (though not uncommon), in that the supposedly "rough" background is in fact considered "better" in certain social contexts (it has more "street cred"). It's an example of inverse snobbery.

It's not clear from the question exactly what kind of "good" image our subject is presenting, what kind of "bad" reality is being concealed, or why she's doing it. Either or neither of the above might apply (but probably not both).

Other possibilities are cheat, liar, deceiver, imposter, pretender, fake, faker, fraud, sham, decoy, defrauder, dodger, double-crosser, double-dealer, trickster, charlatan, two-faced [person].

Personally, I like duplicitous (deceptive in words or action) for general use. But if it's a woman pretending to be sexually modest when I know she's not, I might call her a closet sexpot (a term of my own invention here, but which I'd expect to be understood by anyone I might say it to).

A poisoned chalice is something which seems good when received but actually does great harm.