I somehow always imagined that Mon Mothma was not onboard any of the ships of the strike force that attacked the second Death Star, as the mission was hazardous and potentially loosing not alone the main part of the Rebel Fleet, but one of the Rebel movements top ranking leaders, would be a double blow, one should not risk.
But as there is no evidence fore or against it, I will let it slip for now
Newcanon Junior Novelization of RoTJ (Beware the Power of the Dark Side) specifically states she wasn't present unlike in Legends (I think it was The Truce At Bakura that showed her being present) - her ship did not jump into hyperspace with the others - but awaited the news - in case they failed and a rekindling of the Rebellion was needed.
Just to argue (because I'm bored), in Legends (The Truce at Bakura) she was present after the battle. Nothing indicated either way if she was there, or arrived after the battle was won like newcanon.
Just to argue semantics 