Mri showed 'mucosal thickening in maxillary sinuses' after concussion, what does this mean?

Probably nothing: This is an incidental finding from your imaging study likely. It probably has nothing to do with your concussion. If, however, you are having sinus problems, i would follow up with your pcp.

Not related: The concussion specifically would affect the brain. Some of the potential findings include micro-hemorrhages and diffuse axonal injury. The mucosal thickening has to do with the inflammation of the sinuses. If you struck your nose or face during the injury, these findings may be from the irritation to the nose/sinues.

The report mentions chronic "mucosal thickening", a sign of prior sinus infections. There is nothing mentioned about a fluid level that might have been secondary to trauma (e.g., bleeding). Thus, an incidental finding of no contemporary consequence, or correlation.