Aeryn, there is a difference between making an observation to oneself for one's own use and voicing an observation/judgment to someone else. In the first instance, you are discovering a useful data point that will help you make better choices in the future. In the second, you may be trying to influence someone else's behavior.
Setting boundaries is something we do for ourselves. Making observations about someone's behavior is a necessary part of the process. When we know what behaviors are toxic for us, we can take steps to eliminate them.
You have set a boundary not to accept someone else's judgment of you. This is reasonable. You are judging this behavior to be detrimental to your wellbeing. Judging a distinct behavior is much more useful than a blanket condemnation, such as "He is a jerk". Setting a boundary with your mother to not allow her to judge you may allow you to continue to have a relationship with her that is more healthy in spite of her NPD.