When we first meet Dot, we don't necessarily get the best impression. First of all, she's crazy jealous of Albertine for talking to Gerry—to the point of attempting physical violence, even though she is six months pregnant. Well, she's certainly scrappy: we'll give her that.
Somewhat awkwardly, she and Albertine end up working together right after that little altercation, so Albertine approaches Dot to clear the air—however, that only ends up making things worse. When Albertine explains that she wasn't hitting on Gerry, Dot takes offense to that, saying, "Why not?... What's wrong with him?" (11.1.15). Albertine seems to decide in that moment that dealing with Dot is impossible, so she throws her coffee in her face and runs away.
Apparently, that is the moment that actually thaws Dot out (perhaps because she was covered in scalding coffee?) and awakens her sense of humor, and she is friendly to Albertine from that point on. So, perhaps she isn't such a bad egg after all, eh? Gerry certainly was fond of her.