It's perfectly fine to use a rigid fork on a mountain bike. In fact, many people do it and love it! There are a few things to keep in mind if you're thinking about making the switch from a suspension fork, though. First, your bike's handling will change. A rigid fork doesn't absorb bumps and vibrations like a suspension fork does, so you'll feel every little thing when you're riding. This can take some getting used to, but many people find that they actually prefer the way their bike handles with a rigid fork. Second, your ride will be harsher without suspension. Again, this is something that takes some getting used to, but many people find that they don't mind the extra jarring since they're not constantly worrying about hitting their pedals on the ground. Finally, you might want to consider upgrading your tires if you switch to a rigid fork. Wider tires with more tread will help cushion your ride and make it more comfortable overall.