I think it only annoys me when people write/text with TOO MUCH capitalization or punctuation. Like...This... or this!!!!!!
I try to write full sentences and use proper capitalization. Like, right now I'm doing it consciously, but I agree that it gives off a "formal" vibe. Especially when texting friends/family, writing Tweets, forum posts, and even sending "professional looking/sounding" emails.
I don't try to start sentences with a lowercase letter, because that would actually take more effort on a smartphone with autocorrect, but I do think using less punctuation and all lowercase gives off a really cool and casual and laid back vibe. Especially on Twitter, which has popularized it. It's almost more natural to "like" Tweets that are all lowercase and relaxed than Tweets that are very "serious" in tone because they use proper capitalization rules.
Casual:
i was at the mall today and this guy jumped into the fountain with his clothes on for a tiktok can we pls not make this a thing lmao
Formal:
Today, at the mall, I saw a guy jump into the fountain with his clothes on for a TikTok. Can we please not make this a thing?