Exactly, music is subjective. To say that one is clearly better than the other is impossible. If you value those particular qualities where May outdoes JP, that's fine. But other people might not. Personally, I couldn't care less about lasting influence because how a culture is shaped has nothing to do with how good a musician is. I mean, again, going back to my example of The Beatles, their lasting influence is immense, but I wouldn't call any one of them the best at their respective instruments.
Uniqueness and character are also subjective. I'm not sure how May CLEARLY outdoes JP in those departments. If by uniqueness you mean he had his own very signature sound, yes, he did, but with any artist that has a unique sound, as iconic as it may be, part of it comes from personal limitations. Look at David Draiman. He has one of the most unique singing voices in Metal, and you instantly know when it's him, but that pretty much nails him down into one particular style, and he'd have a hard time doing justice to certain other genres, because his voice is more suitable for Metal.
With Queen, yes, they were a very, very diverse band, but I would say that's mostly attributed to the fact that their songs were often led by different instruments. Between songs that were driven by drums, piano, or other types of keyboard sounds, if anything, I'd say there werent enough guitar driven Queen songs. Anyway, my point is, Queen's diverse sound didn't come because of May, but in spite of him, and even though he's a terrific guitar player, I'd be a lot more impressed if he had more diversity than that he has a recognizable signature style.
Again, clearly, this is a matter of what a person values the most, and yes, of course most people will pick Queen's guitar player over DT's guitar player because Queen has made more of a cultural impact and became the more popular band. But for my money, JP is a vastly superior guitar player to Brian May. To me, chops and versatility hold a lot more importance than uniqueness or character, neither of which JP is lacking either.