。
You must not make fun of poor people.
Literal
Poor people [object-を] making-fun-of [must-not-てはいけない].
ばかにする ('to make fun of, to mock') uses the suru-verb pattern with ばか (fool). てはいけない is the prohibition form. Slightly different from 軽蔑する — ばかにする implies mockery and ridicule rather than cold contempt.