Weight is just a number, and some chins are just small. When you pick her up, do you easily feel her ribs? If so, she could probably use a bit more weight, but other than that she may just be a small chin. Pregnancy can also really change a chinchilla.
I've had a female who was about 650g that I put into breeding, small for what is usually an acceptable minimum for me. But she was so beautiful, and her pelvic opening seemed large enough so I put her into breeding. She had one litter. But the pregnancy and labor really took a toll on her. She dropped down to 550 at one point, and today hovers around 600. She never recovered the weight she lost, and I had to pull her from breeding. So now she remains a small, but still beautiful, pet chin.
If you are really considering putting weight on your chin, like anything else, it should be done gradually. What type of feed are they on? Are they free-fed hay? Do you feed them any treats? IMO, the best way to gain weight is just to free feed them quality pellets and hay, and usually different types of hay. Treats are just fillers and aren't necessary. Supplement should be a last resort, I think of it as a treat.