Recently I was told that placing a mountainous background on a drawing of a Hippo was incorrect, as nowhere do hippos live in this kind of environment. Is this corredct

No , that is not correct. I lived 25 years in Africa, from Nigeria to Tanzania to South Africa, and saw many hippos in my life in many different background settings.
Hippos do not LIVE in the mountains. They require a big water pool and a rather flat land area directly around the pool in a radius of at least a 1/2 Mile.
But mountains in the background are not incorrect. Just put them beyond that 1/2 Mile distance.

:)