Well, they're regularly mentioned on music sites as being one of the big 'proto punk' banRAB, along with The Stooges and The Velvets. It's usually those three I've seen mentioned together, and in my opinion the MC5 aren't in the same league as those two. They might not be as well known, but I'm talking more about the quality of their music, which I wouldn't rate as highly as many people seem to do.
Allmusic gives Kick Out The Jams 5/5, for example - just one incidence, but there's no way that album gets anywhere near that high a rating from me, hence I think they're pretty overrated in many quarters. That said, Kick Out The Jams is a good listen once in a while, I won't deny that.