Stan Lee introduced some unique heroes such as Spider-Man and Hulk to a couple of generations of fans. He had a significant influence on pop culture.
History will eventually determine how important his contributions are. I suppose Hercules, Odysseus, Beowulf, and similar superheroes of previous generations all had their detractors who weren’t smart enough to recognize the influence those fictional characters would have on Western civilization and world literature. I don’t know whether Lee’s characters will have that kind of longevity, but it’s pretty silly to be dismissive of his impact before it’s measured by the test of time; we already know that he’s greatly influenced a contemporary genre. Not a lot of people have ever been able to do that.