Why M'Ress? Unlike "The Practical Joker" when M'Ress occupied the station because Uhura was trapped in the rec' deck (a proto holo-deck), there was no real narrative necessity for M'Ress to be there instead of Uhura. Nichelle not available to record her lines?
Nichelle played the voice of the Female Ursinoid miner in the episode.
But M'Ress was established as the alternate communications officer. That Chapel was known to lust after Spock was a reprise of TOS but perhaps it was a simple as the writers not wanting two regulars making eyes at each other (they saved that for ST V!)
Maybe Stephen Kandel was kinky for furries? The script had lines for M'Ress, not Uhura.