A small Spanish translation needed.

Where's Dama?

Hey, you're in Fort Worth, Texas. Just stop the next person you see on the street and ask them. ;-)

There's most likely an idiom for that, but I don't know it. Wouldn't you use estar instead of ser?

I don't remember how to conjugate the command form of estar. I'd go with a native speaker for this one.

No soy estupido(a).

ETA: actually, I would personally use"No soy tonto."

That's "I'm not stupid." She wanted "Don't be stupid."
me falta cerebros

Oops, sorry, my brain had a hiccup. Not sure how to phrase that one...

As much as there is in common with Spanish, there are dialects and idioms unique to each country.

What is the origin of the character?

"Me falta cerebros" means I lack brains.

Babelfish is sort of right, but "No sea estupido" is the formal male command form. So it's what you'd say to a guy you don't know very well. Also, estupido, for some odd reason, is much more harsh a word in Spanish than stupid is in English.

So if your character is talking to a friend who's a girl and means something more like "don't be a bonehead," it would be "No seas tonta" (a less paint-stripping word for stupid, like dummy, that you'd use when irritated but not furious) and if he's talking to a wife/girlfriend/friend he doesn't mind viciously attacking, it would be "No seas estupida."

If your character is talking to someone he still doesn't know well or his boss, it would be "No sea estupida," but only if he wants to get fired for being so insubordinately harsh.

I'd probably go with "No seas tonta." Good luck!

Ok FWIW here´s my offering.

Here in Spain (don´t know about Mexico) you could say "No seas idiota". My understanding is that seas is a subjunctive here as it is a negative imperative.

Idiota seems to be more common here and more of an insult.

"What, are you an f-ing moron?" could be something like ¿Eres retrasada o que? (retrasada being retarded in this instance). But again don´t know in Mexico.

Hope it helped.

Directly from a real live Mexican, we have: No seas menso.

I assume, since said Mexican was a teenage girl, that this is laden with sarcasm.