Can I write a letter to the USCIS immigration officer who interviewed me?

Yes, you have every right to correspond with the United States Citizenship & Immigration Service.  Sometimes, USCIS asks you to send them additional information about your case.

In our view, whenever you correspond with USCIS (or any other governmental agency), you should (a) keep a copy of what you sent and (b) send it certified mail so that you can prove that it was received.

As with any communication with the federal government, you should be truthful and concise in what you say.  We would recommend against long diatribes or complaining about your case.  If you have questions regarding the content of your letter to USCIS, you should probably talk to an experienced immigration attorney for guidance.