AHas anyone frosted with buttercream swirls as son as they've tKen their cupcakes from the freezer? Is it ok to do this or should I wait for them to defrost first?

Yes, I do.  They are fine.  I've never noticed a difference in turnout.

AIf you're using a pastry bag there's no reason for it not to come out fine. If you were trying to spread with a spatula you might run into difficulties.