RadFormDecorator does not style paragraph elements, they are out of the control's scope. So styling of <p>'s is carried out by the browser completely according to your preference.
If you want to decorate some text elements with RadFormDecorator automatically, you can use <label>, <h4>, <h5> or <h6> tags, however, this may be incorrect from HTML semantics point of view.
Currently RadFormDecorator may be set to style the following elements:
labels (<label> elements - asp:Labels with AssosiatedControlID set)
any container (only background color is applied from the skin)
Let us know if you need more information.
All the best,
the Telerik team