Had Balon a modicum of sense, would Asha have made a better dynastic match for Robb that one of Walder Frey’s offspring(i’m assuming Roslin)?

Absolutely. For Robb, 15,000 men and the Iron Fleet sailing into the harbor at Lannisport as Robb’s army hit the walls would be far more advantageous than Walder Frey’s 4,000 men. Moreover, with the Iron Islands joining Robb’s northern alliance, his western borders become safe and he can begin establishing commercial and naval connections between the North, the Iron Islands, and the Riverlands so that he can move men and materiel between his various domains.

And as for the Iron Islands, they could gain Asha’s land deal (the Stony Shore at least) as a bridegift and negotiated alliance much more easily than in the aftermath of an unprovoked invasion and failed conquest. And given Robb’s success in the Westerlands, it could have been quite possible for them to grab Fair Isle, Kayce, and Lannisport, which are wealthy, fertile, and more defensible from the sea than Moat Cailin and Winterfell.