I would assume that it's up to the blogger to make their blog readable just like me and my overformatting
My one exception is a friend of DP's who has severe dyslexia. She went to South America for six months, and blogged about her travels so we could keep up with how she got on. I endured the spelling because I know her and was interested in how she was doing. For a stranger, my brain would have switched off by the second sentence, the same way it does with semi-literate posts on forums
If it became too difficult to read then I would certainly give up. I'd probably want to know the blogger's circumstances before I let them know it was unreadable. I certainly wouldn't want to offend if there was some kind of issue like dyslexia, but then I presume dyslexia doesn't lead to a complete loss of meaning? rather than mixed up letters that one can generally work out?