A piece in this week's New York magazine notes that "Tom Sachs didn’t bother to get permission from Hello Kitty’s owner, Sanrio, before he installed his enormous Hello Kitty sculptures in front of Lever House." He says "Hello Kitty is so much a part of our popular culture, I don’t think anyone really owns it. It’s something licensed by Sanrio, but I think her spirit and love and purity belong to all of us." There's a good photo of one of the works being loaded in here.
For its part, Sanrio seems to be tolerating the use: a spokesperson is quoted as saying, "You know, there was Marilyn Monroe and Andy Warhol, and then Michael Jackson and Jeff Koons. When you’re an icon, that’s what happens."
Related interview with Sachs here.
UPDATE: More here from Sergio Muñoz Sarmiento, including some additional photos (thank you Ms. Valencia!). One point I'd make is that, while Sergio says "it seems that Sachs’ sculptures .. infringe the intellectual property rights of Hello Kitty’s owner," on a robust enough conception of "transformative purpose" it's possible these could qualify as fair use.