I agree that they should have made the 'forehead alien of the week' just look human. Wasn't it established in some episode of Trek (I forget which one or even which series, but TNG seems to be ringing a bell) that there was a basic bodyshape/lifeform type that reoccured throughout the galaxy? If so then it would make sense for lots of aliens to just look human. Or they could have made a few basic designs, like Vulcan/Klingon/Cardassian etc and then made very slight variations on them. Didn't Farscape do something similar to that with the Scarrens?
I don't think this was a problem confined to Voyager though. Better (or less) alien design should probably have been established in an earlier series, like TOS, or at the very least TNG.