How is Mayella Ewell portrayed as both disgusting and pathetic in Chapter 18?Give examples of both qualities by quoting examples of statements that...

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)