I don't think it's weird at all, and I don't think it's simply aesthetic. All these things (punctuation, capitalization, emoji/emoticons, representations of non-verbal communication like "lol" and "hmm", message boundaries, message send times) are ways of conveying tone, nuance, and a personal voice in an otherwise sterile, flat medium.
These nuances have been around as long as we've had instant messaging, but the specifics have changed over time. Representations of laughter are a good example: over the years we've had (in no order) "lol", "Lol", "LOL", "rofl", "ROFL", "lmao", "lmfao", "LMAO", "LMFAO", "hah", "haha", "hahaha", "HAHAHAHA", "roflmao", "roflcopter", and others. Over time the nuances of these options have changed, with some no longer au courant (the rofl family is currently outmoded), just as spoken slang and language rapidly evolve. Punctuation choices and so on follow similar patterns.
I do roll my eyes a little bit at some of the wild prescriptivism that can be found in this thread, which I think completely misses the point. The footnote from the original post I find especially infuriating:
>sloppy with their written communication, which is to say "careless and unsystematic; excessively casual".
Casual speech is not sloppy! Choices of punctuation, capitalization, and so on are deliberate.