Plas is a difficult character to get right, in no small part because half the writers who handle him seem to think that he should sound like he's being played by Jim Carrey.
As much as I rag on the guy, it was actually Mark fucking Waid, of all people, who did an awesome Plastic Man, once he got past the idea that every word out of the guy's mouth had to sound like a stand-up comedian doing auditions.
To me, the key to Plastic Man is not that he TRIES to be funny - rather, like Deadpool, when Plas is being written right, he should just BE funny, as a natural result of his twisted thought processes.
Much like Impulse in Young Justice, Plas thinks that he IS normal, and it's the REST of the world that's weird.