Bombed an academic interview, should I do anything about it?
First of all, you should cool down before you take any action. Clearly, the interview was an emotionally intense event for you, so don't do anything in the spur of the moment (and "a few days later" may still qualify as "in the spur of the moment" in this context). Let a few more days pass, and see how you feel later.

Second, seek the guidance of a trusted person or mentor who is aware of the details. Maybe your advisor can feel out to what extent you have really bombed the interview quite as badly as you perceive it? Note that especially for interviews on grad student level, the committee will be positively used to people giving bad presentations and not being quite able to argue their case. Unless you have made a really monumental blunder, the committee has probably already forgotten about the details of your interview by the time the next coffee break rolled around.

Which brings me to the third point: no matter what you do, don't write an email reminding people that you did poorly or excusing yourself for not doing better. Don't try to argue your points, either. This won't make your interview better, but now people may also think of you as somewhat unprofessional. If you feel you need to write an email, keep it professional, thank them for the opportunity, and ask for feedback.


  Any alternative action that you would recommend? Or should I just let it go?


Letting it go is generally the right course of action in these situations. I have personally done my fair share of bad presentations, bad interviews, and (in retrospect) crappy paper submissions. Learn from them and improve for the future. You are a student. Mistakes are to be expected. Life will go on.