Don’t worry about weight loss at first – track your progress in terms of miles and hours each week with a bike computer or a smart/fitness watch. Try to do more hours or more miles each week, this will increase your metabolism and you will burn more calories during all activity. Strava is a great tool that I use as a training log, sometimes I’m feeling down on myself and I look back through my training log and I’m like “hell yeah I’ve done so much good riding lately and I’m doing more than last month.”
Basically give yourself kudos for doing bigger and bigger rides, because that is measurable progress and its not debatable. Your weight will decrease much more slowly, and will be highly variable week to week because of hydration, digestion etc, and if you obsess over it too much you’ll feel too many little setbacks that can be discouraging. Getting fitter through mountain biking is a positive reinforcement feedback loop – the more you ride, the more you can ride.
That is: weight loss is your long term goal, but you can keep up your motivation by seeing progress in your milage or time and reaching short term goals.