The thread header is pretty much to the point. Can one or two specific people be credited as having created M'Ress for the animated series, and if so, whom? Can Gene Roddenberry legitimately assume the honors this time (or "take the blame" if you dislike the character), or was it one or more of the other noted people developing the project like DC Fontana or David Gerrold? Was it director Hal Sutherland or maybe the front runners of Filmation, Lou Scheimer or Norm Prescott?
I don't think Roddenberry had much creative involvement with TAS; it was essentially D.C. Fontana's show. As for Gerrold, he was a freelancer as far as I know, not a member of the staff.
In order of production number, M'Ress's first episode was "The Survivor," written by James Schmerer. However, Memory Alpha says that M'Ress is described in the script to "Mudd's Passion" by Stephen Kandel, suggesting that he may have conceived the character. I checked my copy of the "Star Trek Animated Fan Club Newsletter" issue 1, which came out not long before TAS premiered, and it gives "Mudd's Passion" as M'Ress's first appearance. So I'd tentatively suggest that Kandel may have created her.
But it's also possible that she and Arex were developed in-house by Filmation's staff and Fontana as they put the show together. Sometimes in animation it's an artist who creates a character, with the writers then developing them based on the initial artwork.
Also, was she intended to be anything beyond a token alien? If more episodes had been produced, was anything akin to a M'Ress focused story considered?
That doesn't seem too likely, but if someone had come up with a good idea for a story centering on M'Ress or Arex, it could've happened.