Is it bad to go on a low sugar diet for Anorexic recovery? My current BMI is 17.7 which is an increase from 15 point something.

Well rounded is best. A high sugar diet,is really not the best choice,for anyone. While some sugar in your,diet is ok, this should be in the context of a well rounded and nutritionally balanced diet. Work with a nutritionist to figure out how to best do this.