Cindy isn't a human but a Fable. That said, what she really is isn't very well explained either. Also, she's Cinderella--immortal super spy and she lives by a very strict set of rules that doesn't allow her to reveal what she really is. She goes around not thinking the F word a lot because damn you Camp and Camp's telepaths. Cindy doesn't have any mental shielding canonically unless they did something magical and never informed the reader.

You already know what the witches of the Discworld think about other people trying to control them mentally. Tiffany is very prickly about mental invasions and she can see/feel/sense people messing around in her head. She will tell anyone who is doing so to sod off--politely at first. She is probably more open to having a mental conversation than Cindy.