Many of Mayweather's opponents may not have been the very best of the division or at the top of their game, but the vast majority of them were good, if not very good fighters. The same can't be same for the Klitschkos' opponents.
Lewis actually got a lot of shit for his opposition when he was the champ as well. And for being boring. He was never all that popular. Still, from what I understand his competition was still better.
As for Vitali specifically, he's not quite the bore that his brother is, but his style is just ugly. When Mayweather's fights are one-sided, you get to see a master technician at work. With Vitali, you get to see a big and smart but rather awkward and robotic fighter bludgeon someone.