If character traits aren't useful at all to classify a character, then why do you consider, for example, Elizabeth Schaeffer a Mary Sue, and not Evans? Why is Elizabeth one?
As I've been saying, "Mary Sue" was coined to refer to a certain character type
done badly, not just a certain character type. More than that, it refers to the practice of writing the
main characters badly or out of character in response to the Mary Sue. Schaeffer is a classic example of that. Kirk is out of character in the way he falls for her and romances her. Sure, he had a lot of romances, but the
way he expresses it in
Death's Angel is quite bad. Not only that, but Schaeffer
herself is written inconsistently, because she's supposed to be this super-tough badass secret agent, but when Kirk seduces her in the most disgustingly condescending, infantilizing way I've ever seen in print, she melts into his arms rather than kicking his teeth in as he deserves. It's just bad, unbelievable writing, and that, above all else, is what defines a Mary Sue. It's not about
what the character is, it's about the poor quality of how they and the people around them are written. (As I mentioned, that's why Tremain in
Vulcan! also fits, because Spock is forced to be out of character and stupidly wrong about something obvious so that Tremain can be portrayed as smarter than he is.)
On the other hand, I never had the impression that Evans had much business in the story to begin with. She took an inordinate amount of page space, just for characters to fawn over her, obsess about her (like Spock) or talk about her whenever she's not present. Evans was there just to be the center of attention, but the story could have worked the same without her. She was much more of a gratuitous insert than Schaeffer.
Once more, there is absolutely nothing wrong
per se with a story where the guest star is the center of attention rather than the leads. That was a commonplace norm in the era when TOS and the Bantam novels came out. It's exactly what Roddenberry intended
Star Trek to be when he pitched it as "
Wagon Train to the stars," a reference to a long-running TV series known for its pseudo-anthology format centering on guest stars of the week, so much so that nearly all of its episodes were titled "The [Guest Character Name] Story" or otherwise referred to the guest character. Naturally a story written to highlight a guest character is going to center on the guest character and have the main characters mostly talking about them. Heck, look at
The Fugitive or
The Incredible Hulk. Only the occasional episode of those shows actually centered on the main characters' own goals and relationships; usually, the plots of the week revolved around the guest stars, with the main character being there to support or catalyze the guests' storyline.
I also cannot agree that a mischievous, cute, enigmatic trickster figure is out of place in a novel about cat people. More seriously, though,
Uhura's Song is a book about people having to challenge their expectations and learn to see things from new perspectives, and a trickster like Evan is a good catalyst for that.