I can think of better choices for the ceiling. Like paneling...
People mostly use that material for the outside of the teardrop, covering it with a coat of plastic paint to give it some UV resistance.
You could use it as a ceiling, but it's a bit brittle. You might want to drive screws thru some kind of a trim, and then thru the FRP. The trim would give it the reinforcement, so your frp doesn't break thru the screw heads.
Here's Buford's frp Grasshopper...
and I don't know what matl he is using on the inside... it could be frp...
but his galley is definitely frp.
Mike...