Outsold the top books? Of course not. But the Walking Dead was #31 in May (50k copies sold), #47 in April (37k copies sold), and #54 in March (34k copies sold) according to ICv2. I think May was a big issue storywise, but it regularly outsells a lot of Marvel and DC monthlies. In 2010 (before the show launched) the book was averaging 10k copies less a month. I guess 10k is not a huge number, but it's almost 50% for a series averaging 25k copies a month, and is a pretty big bump for a independent book that is close to 10 years old and nearing 100 issues.
11 of the top 30 graphic novels on Amazon are Walking dead TPBs. 5 of the top 30 are Batman related (including Earth One which is out tomorrow, and a visual guide to the movie). 4 are something called Dork Diaries, 2 are Diary of a Wimpy kid, none are Avengers related. Local HMVs sell graphic novels where I live and the Walking dead has more shelf space than all of Marvel. DC does seem to be doing better in securing real estate outside of comic shops for their Graphic Novels, so kudos for them.
I know that the Walking dead show is popular, and that having the entire universe contained in 100 issues makes it easier to manage, but the Marvel movies have made billions of dollars in the past 5 years. I don't think they are doing nearly as much as they can do to leverage that exposure.
EDIT: Also, the last time a new volume of the Walking Dead trade paperback came out, it sold over 20k copies in the direct market. Volume 1 sold 20k copies in the two months after this season of the show wrapped up. The book is making a lot of money for a non-Marvel/DC event comic.