Does it matter?

For sure, there was for a time a serious interest in making this sort of judgement about Hong. For all intents and purposes, serious academic history on the Taiping begins after 1912, when the overthrow of the Qing allowed study of the movement without sanctions, and when generally pro-Taiping elite intellectuals were able to sponsor study of their claimed intellectual antecedents. For nationalist and Marxist-leaning historians of this period, Hong was a visionary who recognised the plight of the Han Chinese peasantry at the hands of the Manchus and the gentry landlords. Jen Yu-Wen, for example, very much took this point of view.

But it does us very little good to apply these sorts of blanket moral judgements to a single historical figure. What Jonathan Spence does in God's Chinese Son is pull away from such charged statements and simply try to work out Hong's psyche on his own terms. Whether he succeeds is up for debate (I personally think it's broadly compelling, albeit with some reservations about his version of the visions), but the key thing is not to ask whether he's 'insane', it's to ask why he thought what he did and how that influenced people. Those going for a bigger-picture approach, like Richard G. Wagner in Reenacting the Heavenly Vision and Joseph Esherick in the epilogue to The Origins of the Boxer Uprising, prefer to look at the response to Hong rather than Hong himself. For those looking at the geopolitical side of things, like Stephen Platt, Hong Xiuquan may as well not exist at all, given his general aversion to employing his nominal political power after 1856.

Basically, it does us little good to assign categories like 'lunatic' or 'visionary' or whatever, because ultimately, it's what he specifically believed and propagated, not how we might categorise those beliefs, that mattered.