It is not illegal to cry on the witness stand in court.
It's perfectly fine to cry on the witness stand or cry in court in general.
Crying on the witness stand can actually be an important part of conveying your story when on the witness stand.
Although you should strike a balance between expressing any emotions you have and communicating the facts of your case.
There is no law that makes it illegal to cry on the witness stand and you can cry if you need too.
Crying on the witness stand may elicit sympathy in some cases and can potentially affect how the judge perceives you although it's not guaranteed that crying will persuade them.
Judges mainly focus on the facts of the case and the law and any arguments that are presented by both the prosecution and the defense.
Witnesses taking the stand in court to testify must also swear that they will testify truthfully.
Judges also generally administer oaths to individuals taking the oath.