Yes, it is. This is a court of law. Do you go to church in a "God is dead" t-shirt? Would you go to your daughter's high school or college graduation in a t-shirt?
This isn't a protest, it's civic engagement. The gun control crowd, including the AG, is trying to depersonalize these issues by framing them as challenges to the firearms industry and 'gun lobby'. To the extent that her actions are effect to Massachusetts citizens, she wants to marginalize those individuals as a no more than a small group of despicables made up of swamp-dwelling, mouth-breathing, misogynistic, homo-phobic and racists white male insurrectionists. Show up to court in a 'Tyranny' or 'Heil Healey' t-shirt and you just play into that narrative.
No, don't 'rally'. Come to the courtroom by 2pm, find a seat, listen to the arguments and learn. I'm having trouble getting some of the documents uploaded, but I'll try to post an outline of the case before the hearing.