I would say it's both - he's lost it, and he is stuck in the past. He's similar to Claremont in that regard, although I think Claremont is a far worse perpetrator because he's fucking awful and he's currently, for some incredibly stupid reason, in a position of power again as far as the X-books go, and is doing what he can to retcon all of the GOOD things Morrison accomplished.
Great talent in the past - but that's the key word, isn't it? The past.
And might I concur that the new Doom Patrol is absolutely atrocious.