Does Misty's Psyduck ever evolve?
No it doesnt, its possible she could appear in later episodes where it may be evolved but there is no way to tell now.
but I could be wrong.
we saw misty last in hoenn and ash is currently in unova, so he could have evolved.
they also once thought that he evolved in the orange islands, but it was just a fake.
so if you want to count that as an evolution...