Yes, you should cut down the pellets. She only needs a maximum of 1/4 cup a day, fed once and when they’re gone, she’s done. After that she will turn to her hay for more food if she’s hungry.
I find that a cheap measuring cup with a handle that is exactly 1/4, filled but not over filled, is a great way to scoop out pellets and dump into a dish. That way I know exactly how much she’s getting. They will eat hay last unless they love it, so you need to cut back on other foods and treats to get good habits started. You want those 10 lbs to be muscle, not fat.
Sounds like all is well post-spay today. She will dry out more in fresh litter, too. She’s probably licking the incision site. Look for sitting in a meat loaf position, squinting eyes and grinding teeth as indicators that she needs some pain meds. She’s probably out of pain by now, but you can always give her one small dose of pain meds today to make her more comfortable. Her appetite will come back gradually, but if the hay is there, she’ll eat it.
Btw, did your vet use a different hay that she liked better?