Does anything else about her indicate that she's autistic? She's also not a savant-level bassist, she's just talented. She's described as stupid because she's stupid. She only has enough room in her brain for one thing at a time. Much like Yui from K-On!, she's simply an airhead, a ditz. Actually I'd argue that of the two, Yui is far more likely to be autistic because she does display an unnaturally good ability to identify tones, and she's not a character I think is autistic. Ryo's certainly not written to have autism. There's no reason to read so much into her. If she were a real person with real person complexity, there'd be more to dig into, but she's not.