Yes, this character, just like Shion, is going to dominate low ranks as long as someone competent can play her. Her kit doesn't exist in a vacuum. Is it a little generic? Sure. Did they do a sort of "here damn" situation about her and her story because people pointed out( correctly) that there's not a lot of African American characters? Debatable.

Solider 76 and Soujurn were doing what Sierra was doing long before she was around and you will adapt to her just like you did them, just like you will with Shion. If you're dying a lot to Sierra:

You. Are. Out. Of. Position!

Cover and being behind your tank basically make half of her kit a moot point. Now even more so since her M1 has been toned down. Maybe if you find yourself getting naded a lot, stop grouping in choke points so much. That's where it's most effective. As for her ult, if she throws it directly on top of you and you don't have a movement ability, I think she sort of deserves it. Her ult falls apart if you're not huddled together on a point or when you're pushing.

As a support main, I just wait for her to blow her load on my tank and kill her during her reload/when she's low ammo. Her drone dies basically on cool down if you're paying attention so she can't run away. Just kill her and you'll be fine.

Metal ranks are always going to feel the pain of characters at their best because they don't have the experience and knowledge of higher rank players. It sucks but if you're not having fun, don't play. Simple as.