I encourage you to re-read this somewhat short scene in the novel and consider how you think she comes across as disgusting and pathetic. Certainly, the pathetic part is much easier to prove - as she stammers, cries, speaks in broken/uneducated English - and generally tells her story as if she is making it up on the spot (or rehearsing a lie, as the case may be).
As for disgusting. This one is a little different. When we think of "disgusting" I think most of us imagine something gross or nasty or putrid. In this case, however, it is almost easier to say, "Why might the onlooker regard Mayella with disgust?" It makes her sound a little less like a wad of goo and a little more like a human who might be dispicable.
The digust comes as a result of a couple things that pertain either to Maycomb's specific time period, or a combination of then and now. One, the obviousness that she's lying. Two, the fact that she engages Tom Robinson (a black married man) in a personal conversation when she is home alone. Three, her very appearance, language, and disposition - which all scream "poor white trash."
A couple quotes (however, you could almost use anything she says in this scene):
Won't say a word long as you keep on mockin' me. (181)
I got somethin' to say and then I ain't gonna say no more. That nigger yonder took advantage of me and if you fine fancy gentlemen don't wanna do nothin' about it then you're all yellow stinkin' cowards... (188)