You've had her for 2.5 months and she's still 100 grams? I would say yes, she is underweight. Do you have a picture of her? At 105 grams she should be eating a 10-15 gram rat pink every five days. Hopper mice are too small for even hatchlings, most do start on rat pinks. You want to feed her 10-15% of her body mass every 5-7 days.