We just put 2013 Q2 SP1 on our development server. I'm not sure if this issue starts in Q2 or SP1.
There is a new piece of CSS in the FormDecorator.css that conflicts with normal operation of a decoration zone.
I have not seen this in earlier versions. Since the RadForm class is put on the DIV for the region that is being decorated, this CSS interferes with normal font family and size declarations that may already defined outside of this region.
I'm inclined to think that this is a mistake...?