the novel. I did not see an adaptation until I had read the novel at least a dozen times (over several years), and I think Mr Collins is more than just stupid. (And even in the novel he is described as clumsy on the dance floor, so I'm not sure that the adaptations are necessarily over-exaggerated.)

Mr Collins has an over-inflated sense of self-importance, and he doesn't listen to other people. It makes him feel good about himself to think he is patching up the family quarrel (between his father & Mr Bennet), but he took no steps toward doing so until Lady Catherine told him he should get married. And it also makes him feel very benevolent that he intends to marry one of Mr Bennet's daughters, but this idea is helped along by the report that those daughters are pretty.

His proposal to Elizabeth is pompous, egotistical, silly, and degrading. He doesn't really listen to her, because he discounts her reasons for saying no.