Let's say it isn't a deal-breaker. If everything gives the impression that the author knows proper English, seemingly writes well enough and the fic sounds otherwise interesting, I don't care much about whether they used a beta or not. If the fic is full of mistakes, I can still hit the back-button, but the odd typo or wrong expression here or there will make me grin and be gleeful that I caught that rather than annoyed.
... But really, for the most part, I'm not attentive enough a reader to even catch those mistakes. So if the fic fulfills the other criteria, I'm satisfied.