I wrote something confusing above- Standing up to use your body wheight as leverage and mashing the pedals to get up a hill is kind of slovenly technique anywhere. You should be using progressivly lower gears as you climb or slog through stuff, and saving the stand-on-the-pedals thing as an occasional power play. Sitting and spinning up or through will give you the conditioning to do it better next time.
On a Mt bike, sitting is relative. Your tail can be two inches off the seat and it's still sitting, as long as the legs are in a deep bend. You need to let the bike move under you, and being totally planted on the seat stops that. A full standing posistion, using your wheight as leverage, is the no no. And you can get stuck there- watch how supple a good rider is even in a standing position, it's never really standing, the knees keep a bend, the body is loose, there's no pulling on the bars. The hands are light.

Lizzy