I don't think you should be that worried ^^ Liking a fictional character doesn't mean you plan on marrying a body pillow with the character's face on it XD She's only 9, if she's just discovered the series it's normal she's all over it ;)
Just make sure she still makes the difference between fiction and reality (cuz i she doesn't then there's a serious problem :s )
she's got good taste. generally, fictional crushes don't last very long in the first place, so i don't think it's something you should worry about. it's pretty similar to idolizing a celebrity, so if you'd be willing to buy her (or yourself) something related to your favorite idol, then i don't see why you can't buy her bnha merchandise.
My niece has a crush on Todoroki from BHA. He's the one that control fire and ice. She's nine years-old and she started watching BHA because of me...we watched together. Now, she's saying stuff like he is handsome and awesome. She wants me to buy her a poster of BHA and the first 3 volumes of the manga for her birthday. I'm not sure if I should feed into her crush. Is it normal to like a fictional character?