Well the Belmonts have long disjoints with their whip. Namely ftilt, fsmash, bair and fair and while I'm not 100% sure on bair, I know fair can be angled up, down or straight. Isabelle on the other hand, largely lacks disjoints with her most horizontally ranged moves being fair, bair and the fishing rod. The first 2 are projectiles that can easily be beat by a whip attack if it's timed well, shielding or by a projectile and possibly even the fire from the holy water. Fishing rod loses to shield and could possibly be punished by ftilt or fsmash. The Gyroid mine probably wouldn't help much either since the Belmonts can attack from behind it with their whip or simply detonate it with holy water or the axe. There is pocket for taking one of the projectiles which could help. Not sure how fast Isabelle is but given that she's based on Villager, I'm gonna guess not that fast. The Belmonts also have a burst option with their dtilt which also puts them above the ground for a moment with the second hit which could go over the Gyroid mine.
I think the positives for Isabelle in the mu come to shine with edgeguarding. The Belmonts have poor air speed and their up special doesn't go too far. Isabelle can attack from afar with fair or bair offstage, dair from above or dash attack or fishing rod from onstage. Mew2king has a video about the Belmonts where he mentions that the whip attacks can tether to the stage but he says the tether comes out 1 frame before the hitbox so even if the Belmonts go to tether the ledge, Isabelle can attack without any fear of her slingshots getting beaten out. I'm not sure how long the fishing rod can be kept out for but having that dangled offstage and reeling it in as the Belmonts (or nearly any character honestly) try to recover seems like a good way to keep the advantage state going. The Gyroid mine is also more useful during ledge pressure situations since it can be placed to cover ledge rolls or neutral getup/attack.
I'm personally somewhat worried that the mu will swing to the Belmonts favour, if only slightly. Sure their recovery is poor but then so is Cloud's and he has positives mus vs most of the cast in Sm4sh. Of course, the game isn't out yet so it's hard to say and I haven't had the good fortune of playing one of the demos yet so someone who has could perhaps give more insight.