They might just get in a panic or try sleeping on the ground at the bottom of the ramp or something, of course. But at least it would save you having to pull them out from underneath the coop.
I wouldn't do it if the area under the coop is the only shelter they have from rain etc while they're outside though. If they're in a run that's at least partly covered, or have access to some other kind of shelter, then they might still be unhappy about it but it shouldn't hurt them.
