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...
The quality is remembered long after the price is forgotten, so build your teardrop with the best materials...