Scout compares Mayella to Miss Maudie because she has one row of red geraniums in their yard. These geraniums are taken care of very efficiently by Mayella, so Scout compares the care these flowers receive to the town's best gardener's care. That would be Miss Maudie Atkinson.
What this shows about Mayella is the fact that she has the capacity to be a high-functioning member of society. She is just bound by circumstances. Given the right opportunities, Mayella could have been much more with great education and moral training. This is one of Lee's greatest comments. What we do is learned behavior. Mayella has very little choice on her circumstances and has conformed to that which her dad has created her to be, not that she wants to be that.