To whom does Scout compare Mayella and what is especially sad about Mayella's life to Scout?
Scout compares Mayella to a character in a book she has read:
Mayella sounded like a Mr. Jingle in a book I had been reading.
She thinks that Mayella does not "have sense" and she asks Jem about this. From the way she speaks, Scout surmises that Mayella may not be very smart. She sounds like this "Mr. Jingle" who is no doubt a character that speaks in the same sort of white-trashy way that Mayella speaks. What is pitiful to Scout about Mayella's life is that she has not gone to school and is stuck at home all day long having to take care of seven younger children. She realizes that Mayella has tried to keep herself clean, but also realizes how hard this must be when she has to haul water into the house from the well outside. She also notes that the younger children are often dirty, unkempt and sick. She is sad that Mayella's father, Bob, beats her when he has been drinking. Read more about it in chapter 19.
I think that you should look in Chapter 19 for the answer -- Scout thinks about Mayella's life in this chapter while Tom Robinson is on the witness stand.
Scout compares Mayella to Boo Radley. She thinks, though, that Mayella's life is even worse tha Boo's. What makes Scout especially sad is how lonely Mayella's life is. She says that the whole Ewell family does not really have a place in Maycomb's society. They are looked down upon by the whole white population. This is why, for example, Mayella doesn't even seem to know what it means to have friends.
Scout also thinks that it is sad that Mayella betrayed Tom Robinson even though he was probably the only one who had ever treated her decently.