Now, we may have given Sonic a bit of a rough time. Let's try a different approach. SonicRanger, I get the impression you would rather M'Ress be something other than a "bog standard" alien, thus your question if there has been any "on screen" irrefutable "proof" stating her ancestors evolved within a different biosphere independent of terrestrial life. If that is the case, what do you wish her to be exactly? And, depending upon that answer, would you care to share your reasons for that particular "background" preference?
I don't necessary "want" M'Ress to be anything in particular. This mostly arose out of a discussion in the XI+ Movies section about the two girls Kirk bedded in the last film, whether they were Caitians, and what the other possibilities were. It came up, for example, that "Caitians" is completely unspoken in all of Trek.
I do, though, tend to dislike aliens based on animals. And a feline from the planet Cait in the Lynx constellation is a particularly lazy example of that. Of course, that could all be a silly set of names by just one TAS writer and Lincoln "We'll Sell Anything" Enterprises, not necessarily what was originally intended for the character.
Other franchises have given us cat-like or animal-like characters, from Cat in "Red Dwarf" (evolved cat), catkind in "Doctor Who" (it depends a bit on the canon one follows, but they are evolved cats who might, like our cats today, not be Earth natives), Rocket in "Guardians of the Galaxy" (genetically modified raccoon), and so forth.
It is the more juvenile media, like "Lilo & Stitch," that tend to have aliens pretty directly based on sharks and other animals, especially common animals like pets.
And maybe that's the answer. TAS was considered a kids' show. So have something favorable to kids like a cat alien.
But really what we've established here is that our knowledge of M'Ress is almost entirely background-info-based. Without that, there's almost nothing to go on about her.
Basically, for all the other assumptions and fanon that we've debated here (for example, Spock as the first Vulcan to be to Starfleet Academy), this one -- that M'Ress is an alien -- has, to my knowledge, never arisen.