No, there's no such thing as "negative" and "possitive" crits, only good and bad ones. Good ones tell you what needs changing, whether it be said nicely or harshly, it's still a good crit, infact harsh ones should encourage you more to show them that you can be good. Bad crits are things like "that sucks!" or "Coming along nicely." or "that's great." None of those are helpful.