In my opinion, the Ewell family would have been majorly disgraced if the truth had come out about what happened between Mayella and Tom (assuming that what Tom said was the truth). Although the Ewells are already pretty far down the social ladder, this would make things worse for them.
If what Tom said was true, Mayella willingly came on (to some degree) to a black man. This would have made her a total outcast. No white man would ever want to marry her after that and people would look down even more on her family because they would think the Ewells were so low that they would allow their daughter to have contact with a black man.
So Bob Ewell really had a pretty strong reason to try to keep the truth from coming out.
Mixed romances or marriages were major no-no's in the 1930s Deep South. Additionally, some Southern states had laws that prohibited this type of relationship. Although the Ewell's were already the most disgraced family in Maycomb, Mayella most certainly would have been scorned for life once the real truth came out about her attempted tryst with Tom. However, I believe Bob Ewell's actions were mostly self-serving: His hatred of the black man was definitely a motivating factor; the notoriety and his own personal embarrassment was probably foremost on his mind--not Mayella's reputation.
(assuming that you mean the truth being accepted as the truth in the trial) I think the truth about May-ella's actions would have had a wider effect than simply on the Ewells. To value the word of a black man over a white man, even when it is only rational to do so, would mean denying white superiority in all cases. It would mean admitting that years of mistreatment towards blacks had been unfounded and wrong. People like to believe that they are inherently good and to admit to all that would be extremely shaming.