The problem he's pointing out is that you've put the fiberglass on the outside. This covers up a lot of the details you worked so hard to build into your pep model. If you 'glass the inside instead, you keep your details and still get the added strength of the fiberglass. Also, when you put the bondo on as thick as you did, it just means more work and materials are wasted when you carve it down to the shape you want. Can you do it that way? Yes, but there's an easier way.
If you fiberglass the inside of the helmet instead of the outside, you only have to smooth out the parts that need smoothing (i.e., the curved parts that are hard to build by folding pieces of paper). If you put the bondo on thin, you won't have to spend as much time or make as much of a mess grinding it all off.
You might be able to create a respectable product the way you've been doing it, but you're making more work for yourself.