Going uphill on a bike, well, just plain sucks. Especially if your parents bestowed upon you an over abundance of fast twitch muscle fibers (raises hand). That works really well on short climbs up to about 3:00, after that, it can easily be an MTB (or road!) OMG WTH.
It also sucks if your low back gets "hangry" at you as you climb. If you're lucky enough to have this happen, my guess is there's a short circuit in your posture OFF the bike that is affecting what's going on once you get on it. If you've got consistent low back pain off the bike: GET IT CHECKED. There could be a bigger issue at play, and it may need something more than a few simple exercises to get things under control.
This can come from shortened hip flexors putting you in to a "swayback" arched low back as well as forward rounded shoulders coming from lost range of motion in the upper back. The long and the short of it is arched low back lengthens the abdominal wall inhibiting its ability to support your low back properly. Forward round shoulders can knock the lats offline and take a few core muscles with them.
Luckily, with the right approach to moving off the bike, you can get more juice to the engine on it. Your core and hips are the key to climbing, get those stronger, and finding yourself on a two wheeled incline can get a lot more
I've seen this just about every time I've worked with a "core challenged" rider. Once we get their core and hips a lot stronger, they usually feel better going uphill.
The first exercise will go a long way to reconnecting your diagonal firing pattern of the core to your glutes. As you can see, pushing into the left knee fires up the obliques on that side, and helps to bring the glutes on the right online that much more.
The second exercise uses every muscle in the back half of the body, or the ones that hold you in place as you climb. It can also help with forward rounded shoulders and reestablish some mobility in the mid back giving you a better foundation to pedal with.
This plank variation can help to reconnect the lats, core and shoulders to work together to keep your back happy in the saddle. The key is to activate the lats by pulling the shoulders down toward the hips to help you own your ribs and optimize core activation.
Stable hips and a strong core are the keys to climbing your best. Target these in your strength workouts, and you might just find yourself having an easier time going uphill!