It's just the animation style.
The only similarities you see are due to artistic style, intrinsic to the cartoon, not to the individual characters. If you compare facial characteristics, there are few similarities. The eyes are the same only because every character in the cartoon has the same eyes. The mouth and unibrow look similar but the cromulon's face is wider so everything is a bit wider, the length of the mouth and unibrow, and the distance between the eyes. The nose, the ears (or lack of), the shape of the head, the hair (or lack of), the brow ridges (or lack of), the complexion are all different.
And their voices are only similar in that they are voiced by the same actor, but that actor is very clearly using different a deeper, slower, less drunken voice for the cromulons than for Rick.