Semi-rambling answer: At the beginning of the series, Rhoda was a frumpy, though not horrifying, young, single, Jewish friend of Mary, and somewhat addlepated, female neighbor, juxtaposed against Mary’s lean, WASP-ish turning the world on with her smilej attractiveness.
Rhoda was tolerable, but no great shakes-the kind teenage boys would fuck, but not brag about.
Somewhere in the series, and I can’t remember when, Rhoda toned down to a slightly neurotic friend, instead of the female “Howard Borden” neighbor that she started out being. And, she started losing weight and getting more toned, and less frumpy. Then, on one episode, the storyline was Rhoda’s weight loss, and how she was starting to look hotter, and had transformed. Neighbor Phyllis said something like “Gee,Rhoda, you’ve lost a ton!” In the 70s, and with Phyllis’ delivery, this was considered amusing. She was now a (semi-) co-equal to Mary, with, IIRC, a better job/career. This worked out to make it a kind of a ‘buddy’ series, which made it easier to steal old plotlines.
In the looks department, she was now an equal of Mary, just, again, more ethnic.
Watching the series, in the late 90s, I was amazed by how spooky Mary looked in the series. Not the hot babe of the Dick Van Dyke Show, not even a milfy-version of the same-more of a sunburned, haggard reflection of what had been. I think that she was living off of her DVDS reputation, and that, along with her acting talent, and the shows writers, made her ‘seem’ more attractive than Rhoda.