Much of what we know about Mayella Ewell comes from hints and supposition. In fact, she only directly appears in the text of To Kill a Mockingbird at the trial of Tom Robinson.
Mayella is a dreamer trapped by her circumstances. She is likely the one growing the beautiful flowers among the dump that is the Ewell yard. This symbolism of beauty among ugliness suggests that Mayella aspires to something better than the dead-end life she lives in Maycomb.
At the trial, Mayella appears as a delicate young woman, representing the image of white southern gentility. The reader can assume that this is just a facade. Scout remarks that she seems twitchy and on edge. If we are to believe Tom's testimony, Mayella is a lonely, abused, and desperate teenager looking for an escape. Due to her circumstances, this escape comes in the form of fantasy. That is what leads her to try to kiss Tom.
In the end, Mayella's escape is achieved through her accusations against Tom. By accusing him of raping and attacking her, Mayella earns the protection of a society that had all but ignored her previously. It brings her to the attention of the entire community who, all of a sudden, feels great sympathy for her. However, Mayella still does not think that she is fully deserving of this type of respect. When Atticus and others address her with kindness and respect at the trial, she feels that they are making fun of her. This indicates Mayella's low sense of self-esteem.
As far as her language is concerned, Mayella speaks with a particular southern dialect typical of the uneducated white class. She deviates constantly from standard English by combining words to say things like "whaddya" and shortening others like "nuthin'." While her dialect is not as extreme as her father's, it still indicates Mayella's class and education.