Mri for dizziness and foggy feeling shows, minor mucosal disease in sphenoid sinus. Minor sphenoid mucosal thickening. What does this mean? Thank you

Probably not. Can be either chronic infection which is less likely or possibly chronic hay fever or allergies. I agree most likely is not causing dizziness and foggy feelings. Further vertigo testing would be necessary to better determine cause of dizziness.
Sinus Infection. The sphenoid sinus is one of many sinuses in the face. Those findings generally go along with a (probably chronic) sinus infection. It's possible that that is what is causing your dizziness, but not highly probable. The changes reported would be mild to moderate. Think of the "mucosal thickening" as "snot build-up". If it's really bad the sinus cavity shows "complete opacification".