No, I don't think Spongebob is a overrated cartoon. Spongebob was actually an underdog cartoon six years ago when it first permired. It didn't get really popular when I started to see interest of the show back in 2001 when season two was starting. I just hope season four, which starts in May will be very good compared to some the episodes which weren't good back in season three.
Still Spongebob is not overrated. I mean not only kids are wearing Spongebob stuff, but teenagers, college students, and adults have interest with the show. The show has made 85 million in the US and over 40 million worldwide especially in the UK where the show is starting to take off.