It's hard to judge a snake's weight when they're coiled up, but from what I can see she looks fine. Be careful what snakes you're comparing her to, as some people over-feed their snakes too.

Maybe this will help you judge her weight better:
https://reptifiles.com/ball-python-c...ealth/obesity/

Since you've only had her for 3 weeks & you're going to the vet anyway, I'd suggest taking a fresh stool sample with you if at all possible, so they can check for parasites. That would be one cause of a snake being underweight, because they're involuntarily sharing their food, & if your snake has ever eaten live or fresh-killed rodents, she could be harboring intestinal worms. Not saying she is, just saying it's easy to check for in their stool, & easy to eliminate the problem if it exists.