The low gears on your mountain bike are good for you. It's good to pedal at a high cadence. Most novice cyclists tend to push too big a gear and this is less efficient. Some pro cyclists still go out on a fixed gear (no gearchange and no freewheel) bike early in the season to get their cadence up.
90rpm is a good peddling speed to aim for - this may seem ridiculously fast, but you should certainly be well over 60rpm.
Comparing gear numbers between bikes is pretty meaningless - cyclists normally talk in terms of the number of teeth on the chainwheel and rear sprocket, (e.g. 53x11 is a fairly high gear) but even this comparison assumes a constant wheel size.
The 'correct' measure of cycle gearing is inches. The units are a bit weird as they developed from the old 'penny farthing' bikes where the pedals were connected directly to the driving wheel and the measure was the
diameter of the wheel. The size of 'gear' you could run was limited by your inside leg measurement so a 60-inch gear was fairly high in those days, though still too low a gear to make rapid progress on a flat road.
To measure the 'inches' of gearing on a modern bike you have to take the wheel diameter and multiply by the ratio of the chainwheel/sprocket, so for a 27-inch wheel with 53x11 chainwheel and sprocket that would be a 'gear' of 130 inches. This would move the bike 130
inches forwards for each rotation of the pedals.