Some of that is expected relative to a road bike. Just as when you ride your mountain bike outside on flat pavement - your lowest gears will be too low to be practical, and when you’re going fast on the flats you’ll spin out your highest gear of 32x11.
First - I’d recommend calibrating your trainer using the Tacx app. Power readings can vary if you calibrate when it’s cold vs when it’s hot. Also - perhaps the cadence RPM’s will be improved?
Secondly - the level of resistance you feel is predicated on several variables - your age, height, weight, but most critically - your Functional Threshold Power number. If the FTP number is set too high - you’ll barely be able to move the pedals. If your FTP is too low - the trainer will not provide enough resistance. You can manually adjust this in the game app, in the Menu settings.
If you set the FTP high enough that your low climbing gears are usable - chances are your top gear will be too difficult. Your speed numbers will probably be unrealistic as well. As long as you’re not doing a race where bizarre numbers would skew the results, it’s probably not doing harm to anyone else.
Lastly - have you tried the off road routes where the increased rolling resistance makes better use of your mountain bike?. We are adding more off-road game features like steering that you might find fun. Be sure to select the Zwift mountain bike in your garage to use the steering feature.