My head-canon for this has always been the universal translator also translated the physical appearance of beings so that you could read their body language. This explains why everyone looks like humans with a slightly different color and head shape. In some cases, like energy beings and giant slimes, it simply cannot translate anything and displays the original being.
This head-canon melts in the face of the actual lore, but it helps me suspend disbelief.
Steven King's lesser known From A Buick 8 is a surprisingly good slow-burner that has one of my favorite depictions of extradimensional creatures to date.
