To whom does Scout compare Mayella and what is especially sad about Mayella's life to Scout?

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.