Feeding too many pinky mice is not good..at most..as a treat a pinky can be given...once a month, though it is far better to feed good healthy meals daily than rely on high fat foods.
Dragons usually are underweight for a few reasons. They are not getting enough food, either because they have a lack of appetite, or just are not fed enough. They are eating plenty of food but either having troubles digesting or have parasites which are sapping all the nutrients from their systems before their body can use them.
Finding out why your dragon appears 'thin' is much more important than just tossing high fat and unnecessary foods at them. For us to help guide you, we need far more information about your dragon:
Age, weight, length, basking temps, setup details, presence of UVB, substrate, are they eating regularly? How much do you feed daily and what do you feed? Substrate? When was the last fecal done, or vet visit? Is it a female dragon? Is it being bred or laying eggs?
A picture of your dragon that is recent is also beneficial, especially if its a good clear picture showing your dragon lying down on a flat surface or basking area. Though knowing his weight and length (nose to tail tip or if the tail has been nipped or tip loss for whatever reason, then a nose to vent length can be used)
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PHLdyPayne