Is it normal to think that religious people are less intelligent?

Perhaps on average but not necessarily on a case by case basis. I'm sure there are stupid atheists and intelligent religious people.

I think this is actually a bad path to go down though. Even if religious people were far more intelligent on average that still would make their arguments no more compelling. Even if Atheists were far less intelligent it wouldn't invalidate their arguments either.

Put it this way. Slavers in the 18th century America were likely far better educated and knowledgeable than the slaves they owned yet that didn't make slavery correct. The slave that yearned for freedom did have the better argument regardless of the relative ignorance between the two parties.