If one sees people downvoting a not-so-bad question, should one upvote it to nullify it in that same not-so-good question?
It's your decision. There are no rules for upvoting except that you can't serially vote/vote maliciously.
In my opinion, however, I think you should not, as upvotes have more weight than downvotes (they are worth more rep).
Also, the mouse over text for the upvote button mentions many things, but not "this post does not deserve downvotes."
In summary, technically you can, but it is generally frowned upon.
Well it depends. Some questions/answers are simply downvoted because they don't follow the basic F.A.Q rules. Some are downvoted because of the content which is totally of subject or misleading.
It may not be a good idea to go against the downvotes in such a situation.
But there are so many different possibilities that we can't simply say you should or should not upvote a previously downvoted question/answer. So it's totally up to you.
Also, it is to be noted that on a site like Meta, some questions can be answered with an opinion which is more likely to get different reactions (good and bad) from users. Take one of my answers as an example :