+1 for you’re just not gnarrr enough!
I don’t think it’s nececeraly a fisheye problem either, as most interviews with mountainbike photographers seem to list 10-20mm lenses as their default as they need to fill the shot with the rider, and often just drop the bike, grab the camera, turn and shoot the guy following them over the drop/jump/corner, so don’t have time to get a long way off track to use a longer lense.
But yea, low angles and getting as close to the rider as possible so they fill the shot, otherwise they become a small rider on a big relatively smooth hill, whereas if they fill the frame then whatevery they’re riding over will look big too.
That and my rule of thumb is the background should be worth a photo on it’s own, if your eye isn’t drawn to the point where the rider is then they won’t be the most impressive thing in the photo, so get them at a point where the horizon, trail, a tree, outline of a boulder, etc intersect so the eye is dran to them. Harder to do on video, but remember most MTB clips will be 2-4 seconds long (edited into a film lasting 3-4minutes). So be selective, it’s the 10s before/after the rider’s passed that make it crap.