It will ruin the handling of the bike.
The front end will feel slow like a stick in the mud, and the unnecessary unsprung weight increase will reduce suspension effectiveness on uneven pavement.
Sport bikes are supposed to have lightweight components. Especially on the wheels.
This somehow became a fad with the custom bike decorators, who apparently labor under the false notion that vintage bikes ever had fat front tires. Maybe Electra-Glides and such had fat front tires. No sport bikes had them.