I just watched Nemesis, the Voyager episode that turned this cliche on its head. That the more human you look, the more civilized and less savage you are.
But in general, it's always the human-looking aliens who are kind and helpful and the less human-looking aliens who are violent and savage. For instance, think of some of the most aggressive races we've seen: The Nausicaans, the Shelliac, them Jem Hadar.
I think you're seeing a pattern that just isn't there.
1. You oversimplify races.
You (thankfully) can't simplify Trek into "good vs evil", save for some of the dumber DS9 propaganda elements (prophets, pah-wraiths, Bajorans and all that junk).
2. You sort examples to fit the pre-established pattern rather than forming a pattern from the examples.
How are the Romulans "monstrous" or any more alien than the Talaxians? Or the Kazons for that matter?
They're just guys with wild hair! If that's monstrous, you must've been cryogenically frozen throughout the 80s.
Are the Tellarites and Rigelians less alien than the Nausicaans and Jem'Hadar? The answer would be subjective, I'd say.
Personally, if I had to select one Trek race that I find repugnant, I'd pick joined Trills, no hesitation.
It might not be visible when they're fully clothed, but the idea that they have a worm* in their chests is "uglier" than any forehead ridge to me.
Should the Xindi reptilians be included because they fit the bill but not the Xindi aquatics because they don't?
Was that Boslic freighter captain treated more sympathetically than the "ugly" Ferengi (hint: it's Quark) she deals with? Not at all.
if you were to repertory all examples and counter-examples, I think you'd arrive at a rough 50/50 split, tbh; one problem with the human brain is that it's over-trained to notice patterns where there are none.
*Yes, yes, it's sentient and you're no doubt thinking of using it as an example of how -I- am biased against non-humanoids; so: pre-rebuttal: appearance irrelevant, need to live inside someone else very relevant.