Yes, celebrities should speak out publicly when personal issues they are experiencing can possibly help their fans deal with similar situations. Celebrity, in itself, offers an opportunity to become a role model to fans and others that are experiencing issues and are possibly afraid to take action. Issues of violence, sexual assault, and other criminal acts tend to have a stigma attached to them, and celebrities can help combat that stigma by speaking out publicly.
This is a private matter and Sarah Hyland deserves to keep it private. She doesn't owe an explanation to anyone and her fans should understand that. If she wants to make a public statement, she can but she shouldn't feel that she needs to in order to be seen favorably from her fans.
Sarah Hyland has made a complaint about her ex-boyfriend, accusing him of domestic violence. This issue is at the forefront of America's consciousness right now. A lot of publicity has been made out of the NFL not publicly addressing the issue until public uproar. Unlike the NFL, Sarah Hyland is a person and she should only address the issue publicly if she wants to, not because of public pressure.
Actress Sarah Hyland should not have to speak publicly to her fans about accusations against Matthew Prokop. Her accusations are now a legal matter that will be decided in a court of law. Since she has not commented on the matter publicly it seems she wishes to keep it a private matter. This is her right and she should not be forced to discuss something she is not comfortable talking about.