Nope. The island was real, those that left on the plane left very much alive, Hugo and Ben live out the rest of their days on the Island. Things really happened. They really mattered.
The Sideways world was the self-created Purgatory of the Losties, where they were able to re-connect by recalling their *real* experiences on the Island.
The conversation between Jack and Des before Des went down into the light brought up the question very clearly as to whether their actions mattered. Jack insisted they did. His father's explanation in Sideways world confirmed this (along with the long sought "Attaboy" Jack was seeking from Dad.)