short answer yes the mountain bike is very good for burning calories.
That said if you can, try locking off your suspension or if you don't have that feature just try hardening it. Put slicks on like Refl3x said and then try some SPD clips and shoes, you should be able to pick up both for 80-100 on t'internet - they allow you to efficiently push down and pull up maximising power. Also work on your technique - try and keep your peddling at a steady RPM by utilising your gears better and ensuring your seat is properly fitted with a 20degree bend in the knee at the bottom of the revolution.
Just a few tips but should allow you to cycle easier...
Edits: Good seating position