Though he can' mimic, it's been shown that he can do alot with that body. Remember how he turned himself into tiny missiles? Frankly, him being able to turn himself into a flesh coat isn't too far from what he can already do. Even if you were to disregard how much he could change his body now. One just has to look back to his original power (which he's improved and surpassed on greatly) to show that he could mimic anything he wanted. You have to think about how he was mimicking, his power was based on what he had seen visually, and thus all his imitations were flawed. In his G1 imitation, it was a lousy one that couldn't even replicate the face, his G3 imitation was incredibly improved, but he still couldn't replicate the control medal, and his eyes were the wrong detail, later his Thancruz and Imakirum were also good, but had flaws. So that tells us that his transformations, were really limited by his imagination, and his original power was limited by his organs. The current aptom doesn't need internal organs unless it's for a practical purpose, like a weapon, as seen by his thin layer body over Guyver. Aptom also isn't limited in shapes, though he might have a size limit. That's my take on it from what I've seen so far.