English-speaker here (UK) - just to let you know that Yay! is very definitely used in a non-ironic fashion all the time as a light-hearted, perhaps slightly jokey, expression of delight. In text, and sometimes even in speech.
I can 'calibrate' a Yay to indicate a negative sentiment based on context, but it's a bit jarring and not even very strongly negative. Examples like "Two more weeks of working overtime. Yay" is obviously a negative use but, to me, that's you indicating disapproval of a fairly trivial inconvenience or annoyance in a rather light-hearted way. If you really disliked something I'd expect a more direct statement like 'damn it' or your whatever your chosen flavour of profanity.
The idea of Yay as a general meme for negativity is very jarring, at least to this native reader, and I'd really struggle to imagine it ever catching on as a result. Hope you found this perspective useful!
