Well, yes, but admittedly she has only been defeated by Kaneki and Furuta, and apparently their kagune is a cheat. Plus, she fights with brute strength alone while they're smart and take advantage of their resources, against Kaneki she probably wanted to make him stronger instead of killing him while against Furuta she was weakened and taken by surprise, and so on.
Is it cheap that she's been used not once but twice as a device to show another character's power? Ehhh... Yeah, I think it wasn't the best way to do it and it does leave a sense of dissatisfaction with how she's pretty much only collected defeats, but the manga is not over yet, and I seriously doubt she just bled to death on the floor. So there's still time for her comeback, after all. And as I wrote in another thread, she has fully accomplished her goals so far, which means she has ultimately won where it really counts.
I think Ishida really wanted to make us notice how he's wiping the stage clean of the old guard to make way for the new, and that's the main reason for his handling of Eto. Off with Arima, off with Shachi, off with Noro, Tatara, Yoshitoki Washuu, and off with Eto as well. Who doesn't even have to be dead, since it's enough that she's been shown to have been overpowered by the two main players on the field.
The second reason I believe was to underline her willingness to sacrifice everything for her cause and ultimately for Kaneki. Which should be telling of her character and motivations, like the desire to be loved, the incapability of loving in return, her issues with her parents and similar other themes of the series.