what happened to Mariko and Yukio?

The DoFP cast was limited to the mutants whose powers made for the best onscreen visuals, who were needed to be there for the plot, who had some an emotional impact for the audience, and/or whose survival made the most sense. Despite those limitations, the cast was very large anyway, but there needed to be some sort of limit. They also needed to provide the audience with the sense that not everyone made it to emphasize the gravity of their situation, by the way.

Xavier was the only one who was absolutely useless against the sentinels, but his importance to the plot demanded his presence and his ability to track/communicate with mutants, plus the sentiment that they all had for him made it logical that others would have fought and died to keep him alive. Partially similar to that is Warpath who had minimally effective combat skills against the sentinels but his ability to detect their approach made him critical to the group's survival so before Kitty started saving them all by traveling back in time, it makes logical sense to surmise that they kept him away from combat and protected him at all costs (whereas, once Kitty's time traveling ability came into play, it was useful for him to act as a distraction by engaging the sentinels in combat).

Mutants like Yukio, though, did not have a power that would make them likely to survive, useful in some way, or allow for cool visuals during sentinel combat scenes. And while Wolverine likely would have done everything in his power to protect her, others in their group probably wouldn't have. Meanwhile, as a human, Mariko would have also easily died if she aligned herself with the mutants so it wouldn't have made much sense that they would have survived to 2023, when the last groups of mutants are barely hanging on. Nor would it have been best for the movie as they would have been dead weight that served no purpose to the overall plot and since their only connection would be to Wolverine, who spent most of the film in the past, the most that would have come out of them is a quick emotional scene where they say goodbye to him and some nervous staring off into the distance. The former was supposed to come from Storm who was written to have a romantic relationship with Wolverine but the scene where they kiss goodbye was cut from the theatrical cut and there were plenty of people to supply the latter.