If the only thing you are feeding her is nori - or dried algae sheets, she isn't getting the right kind of nutrients for her body to thrive. Tangs need a mixed diet of all different kinds of algaes. Try green nori, brown nori, etc. etc. let her munch on some dark green lettuce even. If you can, find someone in your area that uses and algae scrubber and get some hair algae from them to feed her a little. You can dry the hair algae before feeding it. Hair algae is perhaps the most nutritious thing you can feed her. Also, try soaking some of her food in vitamin C and garlic before feeding. It seems to help improve a fishes immune system, which could help her fatten up a bit more.
If you vary her diet more, and try to feed her nutritious foods, she will fatten up some. Heck, if you could try "formula 2" flake food by Ocean Nutrition, (I think) it's little green flake foods, and that will give her something different to taste and eat and be a little more nutritious.
Feeding just dried out foods isn't good for fish. Sometimes you might even have to purchase some macroalgae from the store and throw it in there for a treat. Or you could grow your own supply in a refugium. Every little bit helps!