Given that every high level executive I've ever gotten an email from seems barely capable of putting a sentence together, it's possible that more typos mean you're a better leader. ;)
Seriously though I think it depends. I've seen things that were relatively well worded, but, for instance, 'I' wasn't capitalized at all. Sometimes the grammar makes it pretty clear it's a person for whom English is a second language. Then there's people where, basically everything's a mess. I can generally tell them apart and judge accordingly...
Not to mention that, especially for comments, some people are probably typing from their phone. (Which is totally my excuse if you think my grammar is bad. ;) )