I don't have any problem with Rachel Bilson, but I doubt she's going to be the mother. They're only in season 5, and the whole catch with this show is that by the end of the series, you'll find out who the mother is. Honestly I see this show going on at least for a few more years, so it's very unlikely to me that you can say with any certainty that someone introduced in season 5 will be the mother. It could be. Bilson could be a "fake out" for now, and then end up being the mother, but so could just about anyone else Ted has dated or will date in the following seasons.