Burnt corneal in my wood pellet (Uuni) pizza oven

Use far less cornmeal...or, better yet, use semolina. Dust your peel lightly, so that there is not a large residue on the bottom. It doesn't take much to avoid sticking. I also prefer semolina because it is not noticeable in the final product. Cornmeal sticks to the crust, and add (in an undesirable way) to the texture (in my opinion). Be sure to clean your stone between uses. If you see a build up during a baking session, since you are out doors anyway, maybe a puff of air from a bellows or compressed air canister to blow the extra cornmeal away (obviously, do this safely) could work...but I would switch to semolina and reduce the amount.

First the obvious: Try to work with less corn meal. Making great pizza is an art that needs long training and better be prepared that the gain of knowledge also will come with some accidents (at least for the most of us).

At the high temperatures the Uunis are fired superfluos wheat or semolina also will burn and create a bitter taste on the crust.

Next consider to get the right tools. There are metal brushes available that can be used to sweep out the oven while burning. Another option would be to do a little investment and go for a professional pizza peel with holes in the paddle, so that when you lift the peel and slide the pizza into the oven most the extra semolina falls through the holes and does not end up burnt on the baking surface.