I'm uncomfortable with mass-upvoting people's comments even if they are known to have been mass-downvoted; it might make them feel slightly better, but it's difficult to figure out how much to compensate them (especially if the target has a contentious history with the site, as private_messaging does), and it weakens karma as a signal of a post's reception just as much as mass downvotes do.

If you believe individual comments to have been downvoted below their worth, by all means upvote them by way of compensation. But I wouldn't make a policy of it if I were you; if their karma's been pushed down enough to affect their posting privileges, I might tentatively endorse it as a patch, but that doesn't seem to be the case here.