She didn't get much education, right? Especially since she was isolated for no reason and desperate for love. Nobody told her that she can't judge a person so fast.

She isn't dumb in any way, but I guess since she went from being a mermaid to human, she knew the least when it comes to the human world, considering she had Scuttle for a teacher. XD

The only one that I find stupid.

She has her moments, smart and "dumb", but overall she appears to be of below average intelligence. I don't see how Snow is stupid. I think it takes somebody quite smart to be that adaptable and so easily assume a position of authority over seven grown men.

she was dumb to talk to strangers

I'm dreadfully sorry, Anna. I still find you very smart!

Or Ariel

I don't see how is Snow White winning.

I always thought she was stupid

I don't think she is "stupid", I'm just guessing she wasn't educated, especially about the human world. Are there schools in the ocean? (Apart from schools of fish of course). The only things Ariel knew about the human world were things that Scuttle told her. And this is Scuttle we're talking about...