Supposedly the federation is a lot more diverse than humans with weird ears, noses or foreheads. Just that the old shows did not really have the budget and / or ability to make convincing, actually alien looking species. TAS can get around this by being a cartoon, modern shows use plenty of animation.
But even in TNG is a running joke about the cetacean officers that are always off screen of course. But supposedly there is a whole part of the ship under water where intelligent dolphins are doing their duties as crew members.