It's clear that Tailisa is a noble, so that makes her of a similar class to Robb. And it's clear that she's probably on the Lannisters' side so she doesn't consider him a king.
She may be of a similar class, but also maybe not, even if noble.
I think Westeros (or rather, we in trying to interpret Westeros) really suffers from not having clear orders of nobility like Duke and Earl. I mean, the Great Lords, those who are pretty much kings in their own lands but beholden to the Iron Throne (until Robb and Balon seceded) are clearly a step above families like the Freys, who are definitely powerful but not as respected (as Walder Frey's bitching made clear) compared to those Great Lords. HBO lets us know Rhaegar Targaryen was married to Elia Martell of Dorne, the Baratheons planned to marry into the Starks and did do so to the Lannisters, the Starks married into the Tullys who also married into the Arryns, but of course there's no way the Great Lords could always marry only Great Lords, so if Talisa is a noble, as appears likely, it would be handy to know at a glance if her family are way below the top tier or not, but since they are just House so and so, and referred to as Lord so and so, we can't tell if it would be a problem if say, a Tully married somone from House Hornwood.
One thing is for certain, the political implications of marrying Robb would always have been very high, even without the betrothal to one fo Frey's brood - he really should have had it drilled into him from childhood that he owed it to his family to make the best choice for the House and the North. I mean, I don't know if the show has even stated Ned and Cat had even met prior to marrying (the wedding night must have been awkward, especially with a war for Ned to get back to immediately), but they sure as hell got on with it.