The surest way to get a reputation as a liar is to pretend to be very good. The next surest way is to pretend to be very wicked.

H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]

A Little Book in C Major, ch. 2, § 7 (1916)
    (Source)

Variants:

LlAR. (a) One who pretends to be very good; (b) one who pretends to be very bad.
[A Book of Burlesques, "The Jazz Webster" (1924)]

Liar — (a) One who pretends to be very good; (b) one who pretends to be very bad.
[Chrestomathy, ch. 30 "Sententiae" (1949)]