If you're strictly talking about 1 one 1 offensive talent, Melo is in the top 5 so there is no comparison with Lin there.
But there's no denying a Carmelo led NYK team was on the outside looking in for a playoff spot; Melo goes out and Lin comes in and vaults the Knicks to the 7th seed; Melo comes back and they're out of the playoffs again.
Melo is a horrible teammate as someone already mentioned. He called for the ball against the Bulls last night; didn't get it, and threw a fit afterwards. He reportedly was against the signing of JR Smith and now that creates some more locker room turmoil. During the ASG he was asked about the emergence of Lin and he refused to address it and kept changing the subject. Didn't say any of your typical "it's great to see him grow as a player; he's going to help this team" etc. It was like he was offended that someone would interview Melo and the questions weren't all centered around Melo. He couldn't divert any attention to the team or to Lin; it was all about him.
Regarding Lin's stats dropping off, it was pretty obvious that was going to happen when Melo, Stat, and Baron all returned. He's still average over 20 PER. Really no different than expecting Pierce's scoring average to down when KG and Allen came onboard.