Isn't it clear that Starfire has a low IQ?

She's suppose to be naive if anything, if not ignorant of Earth culture. I have no idea why they made her so dumb in the original animated series and dumbed her down even further in Teen Titans GO! I'm assuming for comical purposes, but I don't see why making her borderline r*****ed is funny (its more offensive if anything). Even in the comics it was confirmed that despite being blessed with more power than Blackfire, Starfire did not possess the same intelligence which is why her sister was able to conquer their planet. Actually, Blackfire has no natural power like Starfire in the comic so the series was incorrect to give her powers (She used tech and her intellect be in par with Starfire).

I don't know if she has a low IQ, but I think the reason for her not knowing how to speak English properly is because she's from a different planet.

As for why she can't understand that Blackfire is evil, she may actually be in denial about it. I mean, Blackfire's her big sister, the one she looked up to all her life. And now she's a criminal.

I'd say she's just naive.