Is it a good idea to email the admission committee after a suboptimal interview?
I think you are overthinking this. A polite email followup to an interview is always fine ("Thanks for the opportunity" "I was really impressed by ____" etc), but it shouldn't be used to make excuses or justify deficiencies in the interview.

It seems likely to me that one of two things are true:

A) You are not adequately prepared for the master's program. This is okay, it doesn't mean you are a bad person, but it means that other people may be selected instead of you, and it may mean that you would struggle in the program and are a better fit elsewhere.

or

B) You are overestimating the importance of the answers to the questions you were asked about X, which may have been geared to measure where your current knowledge is at rather than an exam per se.

In neither of these cases does it matter if you try explain your interview performance after the fact.