All I can confirm about downvotes is that comments having a certain number of downvotes will eventually get hidden. For memes, I really wonder if downvotes have indeed any effect, at all.
Now in the moderators defense however I must say that the IMG flip moderation tools are not user friendly (I know this because I am moderator over the Foreign language stream and quite often I hardly know what I'm doing to get the moderation done there... And since I am a programmer myself, I guess that tells you something). I really wouldn't know how to find flagged memes, let alone that I can act upon them