Aren't you forgetting Yamato's relationship with Ace ? It has nothing to do with Momo.
The bond with Ace doesn't change much about Yamao's character. She already wanted to go out to sea. Ace just pushed that desire further along. And aren't you forgetting that Oda already pulled this exact same thing at the beginning of the arc by giving Tama the same connection to Ace? Ace even promised to take her out to sea with him.
Their bond was created for the exact same reason the bond between Jinbe and Ace was created : to link them to Luffy through Ace's memory.
Sure. And if we hadn't already seen how that paid off then it could've meant something. Luffy very shortly after meeting Jinbe takes a real liking to him, remembers him and then Oda gives them moments together dotted throughout Marineford.
With Yamao, Luffy seems mildly annoyed with her, doesn't care enough about her to remember her name despite being told multiple times and getting corrected (something that's never once happened across any of the straw hats), he ignored her expectations to board his ship and pretty much all of the moments between the two were made out to be gags or insignificant. Those handcuffs, the massive deal that kept Yamao on Onigashima for 20 years was resolved almost immediately and even played off as a joke instead of this huge life changing moment for Yamao that she believed it to be.
The big climax of her arc of getting set free so that she can go on adventures happened a whopping...um...3 chapters after she was first introduced into the series...Or maybe, that's not actually the climax of her character arc and instead the arc that Oda gradually builds up overtime is a better insight into where her character is heading? In other words, the stuff she's doing right now with Momo and has been doing for the last 13 chapters straight.
Another reason the relationship with Ace was relevant is that he also had Daddy issues. And yep, you guessed it, that's something Momo also has in common relating to his own identity. We shouldn't be surprised if we get a moment with the two bonding further as they talk about this.
All of this taking place away from the battlefield. Away from the straw hats. Away from Luffy.