Unfortunately it has become quite common on HN already.
It comes from people growing up on smartphone chats where the kids apparently don’t care to press Shift.
What’s weird though is that modern OSes often auto-capitalize the first letter of a sentence, so it actually takes more effort to deliberately type in all-lowercase.
I've already written an extension that filters these comments intelligently. (E.g., quotes are ignored but if the rest of the body is all lowercase it is collapsed.)