An older snippet in the Telegraph says:
Cameron did not rise to the very top of British politics by skipping through life like Fotherington-Thomas saying “hello birds, hello trees”.
So the implication/reading (of the more recent quote) seems to be that his early speeches were "effeminate" (an essential characteristic of the Fotherington-Thomas character), most likely in the sense of lacking strong[er] messages.
Somewhat similarly, but in more detail, the Evening Standard says
For most of the country, quite reasonably worried about inflation, spending cuts and unemployment, this abstruse project has the whiff of what Bagehot called "London business": the sort of silly, irrelevant scheme that we inhabitants of the capital dream up in moments of self-indulgence. "I worry that all the Big Society and Lib-Dem liberty and electoral reform stuff looks fluffy and out of touch," says one senior Government source. "It's all a bit 'hello birds, hello trees' when the public are having a seriously bad time of it." When the patient is undergoing open heart surgery, you don't interrupt to ask if he would prefer Assam or builder's tea.
This seems to be a more direct link to what Cameron said, i.e. it's probably a reference to his Big Society speeches being perceived as out-of-touch with contemporary problems.