I think Numb, not Lemon, was the lead single from Zooropa, and I hear what you are saying, but while the vibe on Get On Your Boots might be different for them, it still screamed of "This is us trying to write a hit single after the huge success we had with Vertigo." I don't even think Vertigo it that good of a song, but I get why it was popular, and it was good fun when I saw it played live last year. Get On Your Boots was a dud live, though. They tried like hell to get audiences to like it, playing it at practically every show on the 360 tour, but I think a small number of U2 diehards are the only ones who probably hope it gets played again on future tours.
Also, I don't put too much stock into how high their singles charted. Look at Rattle & Hum: Desire and Angel of Harlem both got into the top 20 here in America, while All I Want Is You never came close to reaching the top 40.