Then you obviously haven't listened to Grunge. Nirvana is the worst Grunge band. Just because they're the most popular doesn't make them the best.
Alice in Chains, Soundgarden, Pearl Jam, Mudhoney, the Melvins, Temple of the Dog, Stone Temple Pilots, L7, etc, are ALL miles ahead of Nirvana.
And about your soloing question, just do what everyone else said. Play the main vocal melody as a lead. Whatever you want, though, mostly. The philosophy behind the Grunge solo was to be the exact opposite of the Metal solo. This means no shred, no complexity or technicality at all.