I believe that the ashes would tell the jury that the wearer is Catholic. If there are any Catholics or any Christians in the jury, they may be convinced that his is the side of the angels. Alternately, some anti-Christians may be inclined to vote against his side simply because he has advertised his faith.
The prosecutor works in a county where Catholics are a decided minority and where non-Catholic Christians, like some of the individuals on this thread, might not like the fact that he is a Catholic.
If anything, the prosecutor was hurting himself in the eyes of a prejudiced jury.
However, our judicial system presumes that jurors resolve to put aside their personal prejudices and deliberate on the facts.
The prosecutor, like any other citizen, has a First Amendment right to free speech and freedom of religion.