It is forbidden to say that someone possesses a negative character trait. For example, it is forbidden to say that an individual is quick-tempered, argumentative, stingy, arrogant, etc.
To say that someone is of bad character is categorically forbidden as it a derogatory statement. However, to indicate that someone is of average character (for example, that he does not overlook the wrong done to him) — while not complimentary — is also not derogatory and may be permissible. In common situation where the term “average” has negative connotations, such a statement would also constitute loshon hara.
On the basis of the principle of relative statements, that claim that a person known for his piety is, in actuality, no better than average, is certainly loshon hara.
Sefer Chofetz Chaim, 5 Tishrei, page 58