These kinds of statements reveal a fundamental belief about students that, in a teacher, is toxic.
It's the belief that all students can't learn. That some students are better than others. It's an unconscious ranking and sorting of kids according to outmoded criteria using crass and ignorant language.
All kids can learn. I don't believe there is any such thing as a "butt-dumb" student. I do believe that there are teachers who grossly underestimate their students' capability, or have just not bothered to find out what those students really are capable of doing outside of the traditional narrow factory-model view of schooling.
No student is "dumb." Teachers have a fundamental responsibility to find out in what ways students are smart.