Can a US citizen sue a non-resident alien in civil court?

You can be sued.

Yes, but the rule of the thumb is a civil action is usually initiated on merits when Respondent has assets, otherwise even with a court judgment and no assists such monumental effort becomes a Pyrrhic victory.

Anyone can sue anyone in any court so long as the court has jurisdiction over the person or entity. Jurisdiction is a bit complex, the court has to have subject matter jurisdiction AND personal jurisdiction over the person. If the other person has "ties" to the county where you are thinking of suing, then you probably have jurisdiction. Jurisdiction has nothing to do with US citizenship in the sense that you are presenting it.