You do not want your goat fat when she is being mated - it makes problems in later stages of pregnancy much more likely.
I have found that my Golden Guernsey girls are much more prone to Pregnancy toxeamia than the BT type, who by the way is eating a good amount of dairy nuts, oats and sugarbeet, plus fresh grass and branches, milks currently at about 4 ltrs and does not put on any weight! The GG girls will put on weight much easier, and I have now taken one off all concentrates (only grass and branches), still milk her twice a day (about 2.5ltrs /day)but she will be mated by the end of October. My GG girls put on too much weight on dairy nuts - they get a mixture of dairy nuts and goat mix, and milk better on it too. Both had PT in the spring and it was a real struggle to get them through the last two weeks before kidding. Also seem to hear a lot of similar problems in the GG community.
So if you have a traditional, small boned goat and she is otherwise healthy and milks ok, I would not worry too much about her being too thin.