I don't like Sonny myself, but don't think it's terrible. Whoever said about Girlie, there are plenty of names meaning 'little girl' in various languages (Nina, Wiebke, etc.) that are used in those languages.
German registrars can refuse to register a name if it would subject the child to ridicule, if it's a common object or if it doesn't unambiguously identify the child's sex (not sure if that has changed with the advent of self-ID) - so an Alex or a Kim (for example) might need a clearly boys' or girls' middle name. I remember a court case where a registrar had refused to register a girl as Marie Christin as those can both be boys' names (ignoring the fact that Marie/Maria only ever get used as middle/second-in-double-barrel names for boys).
Edit: one of our dd's middle names is a common name, but using an unconventional spelling (it was after someone) and we were half-expecting to have to provide evidence of some kind when we registered her. Similarly her first name, as a non-German form of a name that exists in Germany. But no problems at all. I think it can depend very much on the registrar.