If you're riding trails regularly, an MTB jersey is worth the investment. Road jerseys are built for aerodynamics with slim fits and delicate lightweight fabrics that tear easily when you brush against branches or take a spill. MTB jerseys use tougher, more durable fabric that holds up to crashes and trail debris, and the looser fit gives you room to move and layer body armor underneath if you ride enduro or downhill.
MTB jerseys also typically have longer sleeves for extra protection and sometimes feature front pockets instead of the rear pockets you'd find on a road jersey—since reaching behind you on a rough, technical trail isn't practical. The more relaxed cut simply works better when you're out of the saddle, maneuvering, and shifting your weight around.
Using a road jersey on the trails is a common mistake that usually ends with a ripped jersey after the first crash. Save your road gear for paved rides and grab an MTB-specific jersey for trail days.