Why?
If anything, the fact that only 20% of the tour is played on Nadal's favourite surface adds to his legacy. Imagine how much more dominant he would be if the % were reversed, or even if they were 50/50 (like it was at one time).
Under the premise that Nadal can't be on the GOAT conversation because only 7 of his grand slams came outside of his favourite surface, then the same can be said about Djokovic because only 6 of his GS came outside of his favourite surface (hard courts). Do you see how re ed this take is?
The fact that hard courts is the surface where the tour is played the most (only because it is the cheapest option) doesn't take anything away from Nadal, in fact it adds to it, because he still dominates in a tour where 80% of the time is played outside his favourite surface.