As long as things aren't too wet to begin with the maple leaves should dry up once you spread them out. Worse thing that can happen is they blow away in this case- they work better as a mulch shredded, which maybe, in the future, you can do with a lawn mower in the fall.

Mulches that stay soppy can be harmful- especially in soils with less than perfect drainage, but usually this isn't a problem with leaves but with stinky, wet grass clippings.