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.
Pure speculation, but I'd say that's the most likely explanation for M'ress and Arex. With the limited budget meaning any "Yes Sir, no Sir" characters would almost certainly be played by the regulars it would be sensible of Flimation to create one new recurring character each for Doohan and Barrett (who would up doing the brunt of the extra work) and giving them something in the way of distinctive personalities rather than letting the writers just create random ensigns as and when needed creating extra vocal work for the two of them creating distinctive voices.
Mind, I suppose the flaw in that theory is if both characters were created from the off then M'Ress should have been a shoe in for inclusion in the "Female characters have to take command of the ship" early in production Lorelei Signal