"You're random enough. What would you do if I just grabbed your mother, threw and acquaintance at you, and told you that that was supposed to be good enough?" If questioned about the "mother" thing, she'd come up with an excuse; no matter how she actually felt about Dr. Cuddy, calling her something like "mother" full-out seemed kind of weird, unless she was doing it to tease or test somebody (especially given her own issues with Ursa, who she still always insisted on referring to as her former mother). But it was a good enough comparison, and that was what mattered.