Is CF an insulator at RF frequencies?
Carbon fiber itself? No way. It's a conductor, but a lossy one.
I would certainly treat carbon fiber poles as being fairly unsuitable for vertical antenna supports unless I actually tested them. Tests aren't too hard though.
Hang a wire from a tree or something and see what changes when you bring the pole right up to it. Most importantly, can you detect a reduction in the received signal? You can probably do a pretty sensitive measurement within a couple of dB if you use a steady local signal source and measure the audio voltage somehow (record with a sound card, maybe even measure with a good AC voltmeter)
You can also check to see if the pole affects the impedance of the antenna.. a simple SWR measurement might tell you something, but with something like a 1/4 wave vertical over a good radial system and a lossy pole, the SWR could even get a little BETTER.
A purely insulating pole will shift the frequency of lowest SWR down a bit. A lossy pole will probably shift the best SWR frequency and change the SWR at that frequency, maybe for the better, maybe for the worse.