What law does Mayella break? What social code does she violate? Which would most citizens of Maycomb consider the more serious crime and why?

I think it is important to consider that a little more is
going on than just lying under oath vs. a black man in contact with a white
woman.

#1: She lies under oath because it is actually her
father who is guilty here.  Obviously we can't know if she does
this to protect him or because she fears him.  A little of both?  This is almost
forgivable by today's standards.

#2: She is not simply a
white woman in contact with a black man.  She makes sexual advances on Tom.  She, in
fact, kisses him.  This is what is so shameful.  And she does it because she is so
lonely, dejected, insecure (something), that this is the only man she can get attention
from.  To Maycomb - it is shameful because he is a black man.  But it it might also be
considered shameful today because he is married.

Even by
today's standards it is easy to see which "crime" would be considered more
serious.