If the many impersonal political marriages in the middle ages were not sinful of the fact, I do not see why a mail order bride would be. That said, one critical difference is that the impersonal marriages from then occurred in a Christian society where it could be safely assumed that despite a lack of any experience assessing their personality and fitness, a potential spouse would take Christian morals seriously (at least in public and for the sake of family life). That assumption certainly does not hold today, in fact the opposite assumption is almost at work. And given the material incentivization of the mail order bride combined with a lack of social, economic, and political controls to incentivize marital fidelity, it strikes me as a generally and radically imprudent way of marrying, even if in a vacuum it would be morally lawful.