I am planning on making a cake that looks like a hiwaiian (cant spell at all) shirt. And I was thinking of using rolled buttercream on it.

My question is, will rolled buttercream ne stiff enough for me to make a collar out of

In my experience, rolled buttercream is really soft and not easy to work with. It's much softer than fondant.

I've only used it for cookies. In order to make it workable, I cut the shapes out on a silpat and then freeze them. Once they're frozen, I transfer them to the cookies. If I don't freeze the RBC, it stretches and tears whenever I try to pick it up.