I wouldn't say that Morgoth is particularly weakened by the creation of Glaurung, since he's able to mass-produce Dragons at little loss.I have mixed feelings about this. On the one hand, it seems logical that he should feel weaker. That's simply how things work for him. On the other hand, showing him becoming deformed and weaker everytime he does something powerful doesn't really work. People will think he hasn't got anything left after a while, that he's finished, and that's not true. It also risks becoming comical. I think the problems with this make good arguments to not showing the creation of Glaurung at all. Perhaps he's a bit weakened but the change is subtle, and he's doing a number of things so we c an't say it's because of the dragon project.