I think that Momo is right to use the word 'judgemental'. May has many negative qualities but she also has many positive ones and, bless her, does genuinely seem to want to reach out to Momo and be her friend. Momo, unfortunately, comes to the table with a set of very condescending and self-righteous prejudgements about May, not automatically justifiable ones either, which creates this wall between them.
On the flip side, when you consider the relationships with their respective humans, May is to Momo a sort of hyper-embarrassing step-sister from the wrong side of the tracks. She didn't ask to have to learn to integrate May into her life but Dale's relationship with Marigold makes it unavoidable. I think that she makes Momo (who goes perhaps to ridiculous lengths to be the 'perfect Robo Companion') nervous and hyper-aware of her own negative qualities including short temper and violent streak.
FWIW, I entirely agree with May's tactics. There are some things that should not occur in public or semi-public spaces and that is one of them. Being doused with cold water is a scientifically-validated negative reinforcement technique for dealing with these.