That’s according to the NME, who quote an episode of Chris Harwick’s Nerdist podcast featuring Stimpy (and later, Ren) voice actor Billy West. Says West: “One day, this scraggly kid came in and said he wanted to write a song for Ren & Stimpy, and it was Kurt Cobain … They [TV bosses] said, ’Yeah, that’s great, and they threw it in the wastebasket.’”
As the NME points out, no one’s confirmed the story on Nirvana’s side. Also, I checked some dates: Ren & Stimpy debuted in August 1991, and Nevermind was the number one album in the country by January 1992. “Smells Like Teen Spirit” went to radio in August of ’91, with the video out not much time later. It seems unlikely to me that Cobain could have come into the Spümcø offices unannounced before he was well-known but after Ren & Stimpy had started airing. It also seems unlikely that he would have strolled in unannounced with a completed song after the band was successful, or that Nickelodeon, which was owned by the same company as MTV, would turn away the frontman of the biggest band in the country. But there you go.