1) The Mets tweeted today that "Jonathon Niese was named Mets Sharp Pitcher of the Month for June. He went 4-0 last month. He will be honored pre-game." Niese may very well be complacent knowing he has achieved a milestone; not that "Mets Sharp Pitcher of the Month" is something to brag about, but the fact he's honored pre-game + being a young pitcher = I wouldn't be surprised if he isn't as "Sharp" as he should or could be.
2) It's 100 degrees out there. Yes, Johan not only survived in the heat, but thrived in it, while Niese has gone 4-0 in a warm June, but being the same age as Niese and knowing the heat has gone to my head (heck, why else would I be writing this absurd post?) I can imagine it will go to his.
3) Dan Iassogna, the 2nd base umpire from Monday's Mets/Reds game who called Rolen's "hit-by-pitch," will be tonight's home plate umpire. Perhaps a smaller strike zone will help him leave an even great impression on the Citi faithful...?
...there ya have it... 3 far-fetched-but-slightly-possible reasons Niese may just have a poor performance tonight...