"Yeah, Tikki can't do that. Or, at least that I know of."

So that was a no go. At least for Marinette. Which isn't to say that Amu couldn't do it if she wanted. It just wasn't possible for Marinette to do, at least as far as she knows, given she hasn't had that experience with Tikki, so she assumes that kwami are not able to do something like that.