I think it’s less of a Tumblr thing and more of a “how much time do you spend online talking to people” thing. Or maybe more of an instant messenger and tumblr just has a lot of that demographic. Most of my friend group is online, and has been for decades now, so I lean more into what the OP is saying because it’s true–it is very difficult to convey tone through words alone, and playing with spelling and grammar and diction creates a more nuanced language. That said it is reliant that the audience you’re engaging with have that same understanding. I’ve talked to some people through text who gave me the same reaction you have, and they were people I didn’t regularly engage with. The in-language probably made sense to them and their friends though.