I have no doubt that BJJ is very aerobic and can be very intense. However, it’s not going to transfer very well to mountain sports because the movement patterns have nothing in common.
To get the most bang for your buck, you’ll want to make training activities as similar to the goal event/activity as possible. For example, I’ve spent six years training for skimo, but it hasn’t made me a better swimmer or cyclist.
Of course, it’s not bad to try different sports, but just be aware that aside from some general strength and the health benefits of being active, BJJ won’t help your performance in the mountains.