Keeping karma fixed, lower % is certainly more impressive.
Not necessarily, given that the average score of comments is positive. If you want at least 300 net 30-day karma regardless of % positive, you just need to post a lot, but if you want at least 300 net 30-day karma and 95% positive, you have to be particularly thoughtful about what you write.
I just noticed that if I hover my mouse over the big green dot with my total karma, it says, "81% positive". Presumably 81% of the votes on my posts and/or comments have been positive.
I checked out the % positive for everyone on the all-time top 15 list:
Average = 90.6%, Standard deviation = 4.93%
So I'm 1.95 standard deviations below average for the top 15. Not only am I at the bottom of the list, we would expect me to be at the bottom of the list of the top 39 users. (Assuming these numbers are representative of the top 39 LessWrong users, which is dubious, and that LessWrong users are "normal", which sounds even more dubious, 97.44% of them have a higher upvote/downvote ratio than me.) I've gotten about 6744 down-votes, a bit more than Alicorn's 6711, but still second to Eliezer's 15225.
How should I interpret this? I could say that I'm the most-controversial poster on the top 15 list, and be proud of that. But if I'd had the highest %positive score, I'm sure I'd be proud of that, too. As long as I'm extreme in some way. Or if I were closest to the average, I suppose I could also be proud of that.
Before checking, would you guess that the top 15 have higher, or lower, % positive scores than most users?