Dottie is the janitor of Merton College, and she lets Dolores stay in the dorms even though Dolores arrives a whole week early. (Time management isn't one of Dolores's strong suits.)
Even though Dolores is also fat, she spends a lot of time criticizing Dottie's weight, "watching the fat wobble on her big ass" (12.54)—so in case we were somehow not clued into Dolores's self-loathing, we see it manifest across Dottie's body.
But turnabout is fair play, so before we throw Dottie a pity party, let's note that she does the same to Dolores. When she finds out that she is from Rhode Island, Dottie says "a fatty like you in that little bitty state?" (12.37), which is a pretty epic burn.
Dottie means this teasing in a flirty way, though (which is decidedly not how Dolores means it). She thinks they can bond over their fatness; Dottie says, "You were a fatty, see, so I knew I could trust you" (12.96). She feeds Dolores cake, too, maybe to keep her plumped up and in the same club. Dolores doesn't see this behavior as flirty, though, until she hears the college girls call her "ten-ton Dottie" (13.41) and spew a bunch of homophobic remarks about her.
This sends Dolores into a sexual-confusion tailspin. She's not a lesbian, so she doesn't think she should hang out with lesbians. But Dottie does her best to catch Dolores, even trying to turn Dolores against Kippy, saying, "She makes fun of you behind your back" (14.30). She doesn't tell Dolores this because she cares about her feelings—she tells her because she wants her to stop hanging out with Kippy and start hanging out with her.
Basically, Dottie is just like Dolores. She's sad and lonely (Dottie lost a son) and she deals with it by becoming manipulative (and bad at it), desperate, and selfish ("Do me first" (15.152), she tells Dolores when she finally gets her into bed). It's hard not to think these women are alone for a reason.