Nat is a lot more curvy and full-figured, and of Latin descent. Riya has a more athletic and trim figure of East Asian (Hindi-Urdu maybe?) descent. Aside from dark hair they couldn't be more dissimilar. But you're right, Nat is a nuke, while Riya is 'da-bomb
Sure there are some differences but overall they both have long dark hair, are slightly tanned looking and have brown eyes. Visually they kinda land in the same'ish area for me.
I would have found it more interesting if there was more variety with the Riya-Character or even someone else entirely. For example someone very light skinned or a lot more dark skinned than Nat, red or light blond hair, shoulder length hair or some other hairstyle that's different from regular long hair, freckles maybe, small breast with a big butt maybe, maybe even slightly muscular, or someone really petite to make Ethan look more masculine.
Basically a visual 180 to add some variety into it like with Nats partners (scrawny nerd, college jock, muscular business man, atheltic man, old guy)