In my experience, if your cockatiel is aggressive and mean, it might have something to do with the activity at hand.
Hormonal behavior could be a big reason for the change in your cockatiel’s personality.
Learn the trick on how to pick your bird up. If you do it wrong, you could hurt your cockatiel’s wings or even cause a break.
Regardless of your side, know that wing clipping can cause your bird to become aggressive and mean.
Negative reinforcement or punishment for bad behavior is not a concept that parrots understand.