In order to have cultural impact that is remembered you have to have a SURGE of impact and it needs to be unique for the time. Katy is consistent but I don't think that's going to make her more remembered. She needs a huge peak in order to have that cultural impact.
Also, Gaga has done things more controversial, unique, and risk taking, which makes her more remembered. Whether she continues to be successful or not, her risk taking and 4 year dominance will make her remembered for that moment in history.