The devs have said that the helmet was slapped on Cullen because they had it available and they needed to cover a glitch with his mouth in the early trailers. They'd almost forgotten they had used a helmet and were apparently surprised to find out people had latched onto it as significant (I watched the interview when someone asked why his helmet had to be removed, and a couple clearly went "helmet? what helmet? Oh!" (or words to that effect, lol.) But some people keep wanting to assign significance to it regardless
Kind of like a dozen other things that have been clearly answered by devs, but some people still want to argue that they have the one true ring! Er, answer.

True. It doesn't help, however, that the devs went ahead and included the damn helmet in Cullen's character kit. So, there is clearly some significance to the lion helmet, since it does seem designed for him and his armor. However, not nearly the degree that the fandom assigns to it.
Then again, Alistair is now a cheese fanatic based on one comment, like Sten and his cookies, and Solas and his frilly cakes. So... par for the course, really.