Who is more trustworthy on the stand? Mayella or Tom? Why?

Tom is more trustworthy because he is telling the truth. Mayella is afraid of her father and defensive.... because she is lying.

Mayella is not trustworthy because of several reasons: one is that she belongs to the Ewell family, who are notorious in the town. We learn about the kids of Ewell family from the words of Scout, when she narrates about the experiences at school. Though Mayella is an innocent victim of the Ewell family's harsh ways of living, she belongs to the infamous reputation of Ewell family. As she is a true victim of her circumstances, she is afraid of her father, to tell the truth. Tom Robinson, on the other hand, is a handicapped person, belonging to the black community. It is his race which only exempts him from being believed. Thus Tom Robinson is the trustworthy person than Mayella.