Evelynn's intended to be an assassin, with a heavy focus on flanking and range management within an engagement. Like other assassins taking out a valuable target should be her primary goal. AP is intended to be her primary form of scaling.
For a long time a more diver like playstyle, with pretty tanky builds, has been her most effectively way of being played though, which creates some thematic disconnect. Performance wise we see her as very middle of the pack when played by experienced Evelynn players and compared to other champions played by people familiar with them. Her average performance is a little on the weak side, brought down in part by newer players going for higher damage, squishier builds that tend to be less successful. Balance wise that means we feel she's in a reasonable, stable spot at present.
Long term Evelynn seems like a good choice for a full update someday, given that disconnect between champion thematics and play, plus the age of her model, visual effects etc. We won't get to her this year, we've got other champs already in the pipeline. Next year she seems like a strong, but not guaranteed, contender for a spot though. If we do assassins as the end of year class update it's likely she'd also get some adjustments then.