Okay, first of all, take the paper towels out of the oven. Seriously, you don't need them for this. Heat, either from roasting in the oven or sauteeing in a pan, will make the mushrooms weep and shrink, and you can blot them afterward if need be. You will be surprised how much of the raw mushrooms' (or lots of other vegetables) mass and volume is just water. When they are cooked, the water is discharged. That can sog up your pizza unless you somehow wilt the veggies first. Another source of sogginess could be your sauce. Since I like my tomatoes/sauce either uncooked or barely scalded, there is no opportunity to thicken it on the stove, so I usually drain the liquid from the canned tomatoes (reserving it in case I want to thin at the end) and stick with the solids. I get it to the desired consistency with an immersion blender. This allows me lots of control over the water content of the tomatoes without cooking them. I probably use a thicker tomato sauce than most, but I don't want sog and I like intense tomato flavor. If you're using a low-moisture cheese, it might contribute grease, but I doubt much in the way of water to the pie. My first thought is the raw vegetables since you seem to have used quite quite a few, and possibly the sauce. Hope this helps.
