Originally it popped up on Niconico video as a joke subtitle version of the infamous scene in the original 1990 series (the first one likely being this, recommending a shitty movie [5 Headed Shark Attack]) that spawned a whole series of "Pennywise recommends" imitators.
Then recently somebody had the 'bright' idea of taking the meme and adapting it to FGO, with Pennywise acting as a metaphorical (well, probably literal too) devil tempting the player into rolling the gacha (Pennywise recommends [CHARACTER X]). The popularity of the meme then exploded even further.
