Is it normal see see light shining through the bottoms of the hulls?
Yes. Fiberglass is translucent. Gel Coat is more opaque and blocks more light. Since only a small amount of gel is worn of... you are fine.
Would it make sense to just add a few layers of fiberglass to the inside bottoms of the hulls? (I figure it can hurt anything and I am not worried about a little extra weight)
Not necessary. Hobie 18 hulls wear more slowly than 16s due to the broad rounded hull bottoms. Rarely does a Hobie 18 need reinforcement... especially considering the small amount of wear to the gel coat seen on your boat.