By all means, throw someone a CD case from the second floor of a building, it's going to be juuuust fine.
I mean...it literally was just fine. Depending on how you release it, it's not hard to drop a light, flat object like a CD case to someone one floor down. It's a little careless when it's something as easy to damage as a CD, but I dunno, maybe she shrink-wrapped the thing for extra style points, in which case even if it hit the ground it probably wouldn't damage the disk.
one of the thing about close-up panels like this is we can't easily see the specifics of the action that took place. Did she lob the thing like a frisbee? Or did Mitsuki walk over so she could drop it straight down?