Why radio jamming was not Germany's main D-Day problem

Could German radio jamming have significantly improved the defence on D-Day?

Not by itself; Germany's deeper problem was positioning reserves and protecting movement bottlenecks against an invasion whose location Allied deception concealed.

A defender cannot know exactly where an amphibious force will land, so it must choose between an expensive continuous shell and a lighter crust backed by mobile reserves. Radio interference was secondary to holding road and logistics nodes and placing armour where it could counterattack. Operation Fortitude and Patton's fictitious army helped keep those reserves oriented toward Calais instead of Normandy.