I don't think Lube is stupid
I think that English is not his first language, and that's why he has trouble thinking of what to say.
I have this friend from France who is the exact same way with his speech, and he's insanely smart.
It's like on that episode where they meet Lola, Lube speaks Spanish PERFECTLY with no confusion at all.
So I think he has trouble with English, rather than being stupid.