Do any quotes from Into the Wild show Chris McCandless as being specifically "stupid?"
The book is written from as dispassionate a viewpoint as Jon Krakauer could manage, so most of the judgemental quotes come from interviews and other people. However, Krakauer makes sure to warn those who might try to follow Chris's example that Chris made many mistakes in his attempts to live in the wilderness:
I bring up McCandless's hubris and the dumb mistakes he made -- the two or three readily avoidable blunders that ended up costing him his life.
(Krakauer, Into the Wild, Amazon.com)
Other judgements tend to focus on Chris's idealism and naivety, and most at least mention his failure to become properly prepared. The book is sympathetic, but there are many other quotes like the one above; it is clear that while many felt a connection with Chris's dreams, they also believed him to be almost on a suicide mission. Without training or taking time to learn how to survive in the wild, Chris was almost destined to die, and so even those who admired him do so without any intention to copy him.