May we use simplistic to mean "simple"?

No.  When something is simplistic, it is simple to a fault, excessively simple.  We once saw an ad in which a company was advertising "simplistic solutions" to problems.  We wouldn't mind having simple answers, but we don't want simplistic answers.  If you want a straightforward answer from someone, ask for a simple answer.  If you want to criticize someone for oversimplifying an issue, tell the person that his or her views are simplistic.