I don’t find any attribution for it. I think it is just a saying, an aphorism, like “If wishes were horses, beggars would ride.” Like any other such saying, I imagine it could be used both sarcastically and genuinely, but I don’t think it can be said in the absence of all irony. For example, if the greatest living painter praised your modest exhibit, it might be said to you as a way of noting the extraordinary weight of such praise, but at the same time even the greatest living painter is no Caesar, so it is inflating him a bit as well.