When Scout is describing the Ewell house, after mentioning the dirt and trash, she says there are red geraniums in pots outside the house that look so beautiful and cared for, it was as if Miss Maudie (a superb gardener) had been looking after them. Scout assumes that these geraniums belong to Mayella. This image complements the one Scout describes when she describes Mayella's appearance--that it looked as though Mayella tried to keep clean on a regular basis, as opposed to the rest of the Ewells who took baths once a year.
These two images combined give the reader (and Scout) the sense that Mayella wants to be better than she is--"just a Ewell"--but it's difficult when everyone in the town only associates her with her last name. This leads Scout to the conclusion that Mayella must be the most lonely person in the world--even lonelier than Boo Radley--because white folks didn't want anything to do with the Ewells because the Ewells are trash, and black folks didn't want to have anything to do with the Ewells because the Ewells were white. So, no matter how hard Mayella tries, she won't be able to rise beyond her last name.
The few slop jars of red geraniums in the Ewell yard suggest Mayella's desire to experience something of beauty in her dismal environment.
While the under-educated and culturally deprived Mayella Ewell is probably not familiar with John Keats's poem, "A Thing of Beauty is a Joy Forever," in which he writes,
Some shape of beauty moves away the pall
From our dark spirits,
she probably understands the feelings expressed in these lines. The red geraniums are precisely the "shape of beauty" in a yard cluttered with junk and a dilapidated house that has the gloom, or "pall," of poverty and loneliness. Mayella seems to try to dispel the sad conditions of her life with her flowers of a vibrant color. Interestingly, the geranium is a symbol of esteem and gentility, and red geraniums traditionally have a meaning that relates to comfort. Perhaps, then, the qualities of the red geraniums are those for which Mayella wishes.
That Mayella may possess the "dark spirits" of which Keats writes in his poem is suggested by her apparent loneliness. When she is on the witness stand and Atticus asks Mayella who her friends are, she becomes angry and demands to know if he is "making fun" of her. In addition, after listening to Mayella's responses, Scout concludes that Mayella has a sad existence: "Tom Robinson was probably the only person who was ever decent to her."