there are different approaches to squatting - powerlifting and big squats are all about building to a 1RM and it doesn't transfer to climbing well, whereas dropping to half the weight and doing 8 reps+ it would be much more beneficial for the bike. I've got a decent squat at the moment but my mtb climbing has suffered a lot in the process of getting to that (and I'm a bit heavier too), but that's my focus at the moment.
cross training is the best - fittest and lightest I've ever been is when I was boxing + mtb, and before that I was trail running and riding and the hills were much much easier. I've never been great though, just better than hopeless