Quadman, probably not a good choice for fenders.
Carbon fiber just isn't flexible enough to be a plastic replacement.
If it's a show quad or something that hardly sees any rough terrain, they'd probably hold up ok.
CF is pretty brittle, so if you accidentally kick the fender while trying to start the bike, you might damage the fender.
If you hit a jump wrong and part of your bum lands on the fender instead of the seat, you might damage the fender.
If you catch a twig or snag some brush on a trail, you might damage the fender.
If the tires kick up a decent size rock and it hits the fenders, you're probably going to have delamination.
Regular plastics bend and deform so they deflect a lot of that energy instead of absorbing it, that's why it's such a great material for fenders.
You don't see lightweight aluminum fenders either...they would be much lighter than current plastics but also a lot more delicate for the same reasons.
It's also not a good choice for axles for the same fact that it doesn't handle impact well, it'll delaminate and structural integrity suffers.
Not really any different than fiberglass except that it has a higher tensile strength, so whatever will damage fiberglass will damage CF in the same manner.
If you can't imagine something made of fiberglass standing up to typical use, then CF is probably not a good choice either.

BAC technologies is selling CF drag axles that are sheathed in aluminum and are slightly stronger and lighter than a comparable aluminum drag axle.
They use specialized tooling for the construction that I doubt Robb would have available, so if you're interested in something like that try BAC instead.
Notice there aren't any full length axles made of lightweight construction like aluminum or CF, that's because they're just not strong enough past that point.

As for tie rods and steering stem stuff, that's a safety consideration and it's probably more than what Robb wants to handle at the moment.
I heard that DC Motorsports has steering stems and tie rods made of CF, but I can't find a website to check their stuff.
A simple wrap won't really net you much of any gains other than looks, and if you're only interested in looks then there's a lot of CF decal stuff that simulates the look.
Meier sells simulated CF fenders, or at least they used to.

Lots of things can be built from CF to replace heavy components on our quads, but the tooling and design make it a more costly endeavor considering the small pool of customers.
I would venture a guess and say that whatever it would cost to build in titanium is a decent starting point for CF prices.
A lot of work goes into structural CF, a lot more than battery covers and hoods, because it has to be layered several times over, spun, sheathed or capped with traditional metals, and in some cases completely redesigned because CF has to be a lot thicker to provide the same strength and rigidity.
Plenty of things are possible with CF, it just takes plenty of money.