Mountain bikes are designed for off-road riding, and as such, they generally don't have a place for aerobars. However, some mountain bikers do use aerobars on their bikes, particularly if they're competing in cross-country races.
Aerobars can help you to adopt a more aerodynamic position on your bike, which can help you to ride faster. However, they can also make it more difficult to control your bike on technical terrain. As such, it's important to weigh up the pros and cons of using aerobars before making a decision about whether or not to use them on your mountain bike.