Does wearing a backpack weaken your jumping technique?
You have more control over weight on you than on the bike, so long as the pack is fitted nicely and doesn't bounce around too much. This doesn't just apply to jumps but all the time, as your bodyweight is part of your control but weight on the bike is passive.  Imagine rolling over a tree root on a hardtail or even rigid forks.  Weight attached the bike must lift over the frame, but if you're off the saddle and riding properly your body and backpack float over the obstacle, barely rising.
Moving a few kg/10-20lbs between frame and body is noticeable. I'm a perpetual mountain bike novice but ride a lot on road.  On the MTB (hardtail)  I either use a hydration pack similar to you, or copy my road kit: a litre bottle of water, tool bottle in a 2nd cage, and saddlebag on the bike.  Even at my level (no big jumps, but the effect holds even for bunny-hops) bike handling is easier with the weight on my back, though it's fatiguing on a very long ride.  This makes sense.  When your weight is on your feet and hands rather than your saddle (i.e. most of the time when riding technically) your bodyweight is sprung on your legs - so is the backpack.  When you jump, you launch your body into the air and the bike follows you.  You launch a backpack with you, but extra weight on the bike has to follow you.