Whether clipless pedals are better for mountain biking is a matter of personal preference. Clipless pedals enable more effective transmission of energy from the rider to the bike by locking the foot into the pedal, allowing the rider to pull up in addition to pushing down. This can result in a more consistent, efficient pedaling motion advantageous for climbing and technical biking.
Clipless pedals take skill and practice to use and master and can be hard to exit fast in an emergency. Some riders may prefer flat pedals instead, for more freedom.
Overall, experienced riders may benefit from clipless pedals for a more efficient and powerful stroke, but it's important to assess your own riding style, experience level and comfort prior to deciding between clipless or flat pedals for mountain biking.