Since the other descriptive used in the phrase is "ribbon-decked," the term "lawn-clad" must relate to clothing or fabric of some kind. "Lawn" is a type of light cotton fabric. The word "clad" means "clothed" or "covered." Therefore, "lawn-clad" means "dressed in a light cotton fabric."

A much less common definition of the word lawn is

"A light cotton or linen fabric of very fine weave."

ETYMOLOGY: Middle English laun, after Laon, a city of northern France

So, lawn-clad means dressed in a light cotton or linen.