IMHO, that manager should NOT have made that comment.
If he accepts that job, then he should do the best that he can under the circumstances or quit.
He should not make rude statements about the patrons, who are responsible for his having a job.
*****
I know a teacher who once taught in a junior high school where many students were, well, less than cooperative.
But if anyone expected him to make disparaging remarks about those students' background, they were disappointed.
He would say something like, "They make my paycheck possible. So I am morally bound not to badmouth them."
What he did do was to eventually get a transfer to a school with students of a different background.