If something has been 'found wanting' it means that someone else has tried or tested that person or thing, and that they have has found them to be inadequate or ineffective.
In this case, it means that people have learnt through experience that there is something deficient in Mary's character. Perhaps she has shown herself to be dishonest or untrustworthy, for example.
Note that this is fairly formal and outdated language which we don't use in everyday conversation. Most native speakers know it from the Bible quotation (Daniel 5:27) "You have been weighed in the balance and found wanting".
(I'm totally baffled by Shavingi and Darryl's answers, by the way.)