No higher than B-B+.
Arguably her damage off single random non-CH is low and to do decent damage either requies a situational setup, meter, corner or an expected CH/Fatal. The strongest aspect of her game is her okizeme, as Lambda does zoning better than she does. Her C series of normals are great and help to dominate certain matchups (Tager, Rachel, Tsubaki). As it lets her both approach and retreat (see jump C) (She also has Lambdas great 6A and 2B)
But, arguably, she does lose against the higher tier (Bang, Litchi, Ragna), which means she is not at their level. It can be hard to get steins out on these characters and use them effectively when these characters have comprable strength normals and great forward momentum. You start to notice that she is missing some of those tools to put her in the upper echelon.
Her mix-up is decent, but you really have to hope and force your opponent to twitch during your pressure, because when facing a more patient, defensive player, you can have more difficulty doing good damage and getting that all important oki. Her overhead is seeable and can be blocked on reaction, so you have to train your opponent to expect certain setups as her strings can become very predictable (she doesnt have as many options when poking up close as Bang, Hazama, Litchi, Ragna, etc.) and easily defended.
Great thing about her, though, is that even while she might not be top, is that she is a real momentum character. Her DP is amazing and is generally a get out of free and reset the spacing card when someone is rushing you down. Although, she does suffer like Lambda in terms of dealing with rushdown, but her DP is much more reliable than gravity. She is great in neutral situations as she can just start crapping out steins from mid-to-full screen safely to simply annoy the opponent (its a good way of doing so =3), adding to the fact that she has several great anti-air (6A, 2C, DP,) options and then she can stop predictable run-ins with C or 6C.
Her game is very reliant on 236D, but getting that KND/DP into it pretty much makes her as scary as she is going to be. It puts you in business at any point in the match. Stein control is probably the next most important thing, as having as least two on the VISIBLE field (and not somewhere up in the sky) at any time will go a long way to making people scared of you. Mu is good, but not great.
HAVE FUN, AT LEAST SHE IS NOT RACHEL!