Because of the relatively upright position in which one would sit on a mountain bike, this windshield would have to come up much more than just a few inches above the handlebars. A monstrosity like that couldn't be very aerodynamic because of the large area exposed to the air. ie., it'll be more of a sail than a drag-reducing mechanism.
Also, a windshield in itself is probably not that good an idea. On superbikes where the rider stays low, the windshield makes sense because it isn't very large and at the speeds they move at, the uneven contours of their helmet and shoulders would make a difference. On choppers(the motorbike, not the aerial vehicle) it makes sense because riders don't want insects and other particles on their suit and helmet while a large windshield doesn't matter too much since the bikes are powerful enough to push the windshield through the air.
In cars, the windshield is not the only thing that provides aerodynamic efficiency. Combined with all the other windows, it lets air flow around the car and not enter inside (if it did, a large amount of turbulence would form and make the car very inefficient).
so for example I could smoke a cigarette on my last motorbike without the wind bothering me much.
Actually that won't be possible, when you move forward, the air just behind the windshield would start getting vacated and air from the sides and top would start rushing in to take its place, this would form very powerful turbulence and your cigarette would extinguish faster than a candle on Mars
Your best bet is do what most people do and duck down as far as you can without your knees hitting your chest and with your forearms on the handlebars so you don't end up bending your elbows awkwardly and looking like a 10yr old who thinks he can break the sound barrier on his cycle.
But, if you have the cash to throw around and you only ride on road, do yourself a favor and buy yourself a road bike. Or if you don't have enough for a new cycle but enough for some upgrades, do what i did and swap out your handlebars for drops and get thinner wheels as well. [I did it to my Trek 6500, a 20yr old hand-me-down]
On a more hypothetical note, and if you're hellbent on some sort of perspex contraption, you could try designing a full cockpit-style egg shaped capsule for yourself, now that will help you go fast, and of course smoke that cigarette.