Your basic premise that one can see better with the glasses tilted is false. If the lenses have the right correction for your eyes, then tilting them will make things worse.The reason this does work often is that the lenses are not at the right correction. In young nearsighted (myopic) people, the myopia usually gets worse with age. The glasses may have had the right correction 2 years ago, but meanwhile the eyes have gotten a little more myopic and a stronger correction is required. Tilting the lenses makes light pass thru them in a way that effectively makes the lens seem stronger at that angle. If distant objects are a little blurry due to more myopia than the lenses are correcting, then tilting the lenses will make horizontal edges sharper. It does nothing for vertical edges. But, overall the image will appear sharper.
This is a common optical phenomenon that when you tilt the lenses of the eyeglasses, you can see objects more clearly through the lenses because of the higher refractive index of the tilt lenses. You had better go to the ophthalmology to have your eyes examined to see whether your glasses prescription are not correct or your eye vision has gone poorer or not.
Glasses are all optical devices. when you tilt your glasses, it's possible that a phenomenon called "Prism effect " would appear. That effect could change the appearances of things in your eyes , and if the angle you tilt your glasses is just appropriate , you can see more clear .
In another hand , there could be another fact the situation you described suggests, that you should have a further examination for your eyes and as well as the glasses, to find out whether or not the glasses are still suitable for you . The damage of your eyes could be horrible if you keep the unsuitable glasses on .
This is a real effect, but it doesn't have anything to do with coma or any of the optical aberrations. It is caused by the fact that the effective focal length shortens as you tilt a lens. When your eyesight gets worse, you need a stronger focal length lens in your eyeglasses, and tilting the lenses has this effect. The problem with doing this all of the time is that it introduces distortions such as the coma you've pointed out in your question. Read more at http://physics.stackexchange.com/questions/71055/why-do-nearsighted-people-see-better-with-their-glasses-rotated