Question on cycling

I've been doing quite a bit of cycling lately, on a mountain bike that is obviously not suited to road biking..

I can't afford a decent road bike..

It gets very difficult road biking on my mountain bike, with all the suspension and everything absorbing my efforts..

So my question really is.. is this more beneficial for me for fat burning due to the increased efforts to go a certain distance? Like, would I burn more calories cycling on my mountain bike on the road as opposed to a road bike

[Giant]

Over a fixed distance... Yes.
Over a fixed time... Probably not much in it.

[Refl3x]

Buy some slick Tyres for your mountain bike-- you`ll be amazed at the difference
however its also worth noting considering you are mountain biking for fitness/fat loss wether its harder on big tyres or easier on slicks doesnt matter-- what matters is your fitness and burning calories and you will find it harder and therfore more productive on bigger tyres

[daleinthedark]

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