Context matters. It doesn't really say much on its own.
If it's surrounded by figurines or there aren't any other interests displayed, it does make you seem a little absorbed in it. Also, it matters whether this is just like on a standard-sized piece of paper on the wall or if it's like a jumbo poster. However, if you have other posters up with other things, it'll make this look like it's just another one of those things. Additionally, it'll be more fine if it's over like a tool space or a proper art station or a guitar with peripherals, something to make it look like it's adding art to a space that's already meaningful instead of the display itself being the meaning.
She also has a little story, about how she's shut herself off from of others, about keeping herself forgotten, about the horror of having lived as she has, about maintaining the potential for something more in spite of all this, and having an interest in her story gives an added context to her display. Liking a character as a character is plenty enough reason to display them in your room.
The thing that'll get you judged is coomer shit, unironic waifu shit, or anything that makes it look like you're just fixated on that one interest. If you're worried it won't come off as wholesome, you can try adding a fumo.
Finally and most importantly, the person who saw it was your sister, so just ask her what she thinks or if she thinks that it reflects any significant judgments about you. If it does, you can ask if it's just that one thing by itself or if it's that you have several of them displayed, since it does seem pretty harmless if it's by itself.