Is Tank Girl supposed to be mentally retarded?

I think every character in this movie is...

Well, that either's mentally impaired or mentally ill. If you wanna say retarded just say retarded and don't frickin' try to give some legitimate weight by adding "mentally" - which in this case is both wrong and unecessary.
Aside from that, I've never read the comics but I have a feeling Tank Girl wasn't "retarded" in that one which makes your whole post both pointless and useless. Reflects on you I figure.