If you watch Star Trek, you might notice that most of its aliens are basically humans, with ridges or spots somewhere on their face. The animated shows have a bit more variety, with "anthropomorphic animal" aliens, but still stick to a certain body shape. While this decision is unrealistic - aliens would probably be completely unrecognizable to us - I think it actually strengthens the show, as it makes the aliens easier to humanize, helping to express the franchise's overall message of unity in diversity.

There's even an in-universe explanation for some of it - apparently, a now-extinct galactic civilization felt lonely in the universe, and seeded their DNA on several planets four billion years ago, after which life evolved on all of them in roughly the same way.

Come to think of it, one major mistake that was made in a recent show is Strange New Worlds' depiction of the Gorn, changing them from anthropomorphic reptiles that fought the Federation based on a misunderstanding, to purely evil Alien-esque horror monsters. In the process their design changed into more of slimy reptiles. But I think they were way more compelling in their original appearance, because they were humanized in the end.