Just last week I had to implement a common graph algorithm from scratch to solve a business problem (basically topological sort, with a minor variation). It's certainly much harder than reversing a binary tree (which is one of the easiest possible interview questions you could imagine).

Generally it wouldn't really make sense to reverse a tree in practice (why not just build it the other way initially?) but it has a similar structure to other tree traversal things that could actually come up so it's reasonable to ask.