No its not. It would be horrible decision for the product since there are too many heels already but for christian's character its good. His face run has completely worn out, they jobbed him out too much, 100% depleted. People barely react to him anymore, entrances or during matches he doesnt get much support. This heel turn could make him interesting again and Christian damn sure is good enough to get heat from any crowd. Infact, he needs this. Guys like Christian, chris jericho etc.. always work best as heels.
There's not even one good reason to turn Christian heel. It would make zero sense. As noted, nobody wants to boo him, there are too many heels as it is, his only potential feuds in the near future would be with Brodus and Ryback, and he wouldn't get to be the kind of heel he was during his first WWE run and in TNA. Plus he's considerably better in the ring as a face, which makes him far more valuable as a veteran.
Maybe some of us are reading too much into it, but saying that a move should be banned is not something a face does, especially in his home state/province. (Of course, all but the top faces walked out on Triple H last year, so who knows.)
If they're turning him because of the lukewarm reactions he was receiving during his most recent face run, then that's ridiculous, as a) he'd get even less of a reaction as a heel; b) he still got the crowd into his matches, which is a testament to his likability and shows that they're willing to support him; and c) his lack of heat was Vince/Creative's fault. He never had this problem before. From his return a few years ago to his (ill-advised) heel turn, he was one of the most over faces in the company. Then he turned into a generic whiny chickenshit, lost all his matches to Orton and Sheamus, and had Teddy, Triple H, Bret, and Edge all calling him a bitch. You can't just bring that character back, start booking him against heels, and expect everyone to get behind him again. That heel run did a ton of damage to him, and he needed to endear himself to the fans again. With next to no mic time, little ring time, and no storylines (the Cody feud started pretty well but went nowhere), he wasn't given a chance to do so.
And when a guy doesn't matter, the fans pick up on it and react accordingly. You're perceived as you're presented. In his last month or so, he was neither a top face nor a comedy act, just a guy coming out for a short match he was probably going to lose.