Sources on that? I frequent a lot of wrestling sites and have seen no reports on the Rock being asked to "tone it down".
Also. Of course it is conceivable that Rock can "lose it". He might not be in sync with the current product, with him being from another era of it that is very likely, since he is used to working a certain way. When Rock was in his hayday he improvised a lot of his promos, now he works with a private writer writing them for him. He also isn't as used to working with the people he is working with now. When you work with people more, you get more comfortable working with them and thus can improvise, try new things and help each other improve more.
Rock can loose his touch, but that does not necessarily mean that he is bad, just that he needs to get in sync with the current WWE product.