The Complete Musician by Steven Laitz states that melodic intervals larger than a fifth are considered dissonant and should be avoided. It also states that two leaps of a third are fine, provided you change direction afterwards, which Mozart does.
In Mozart's Sonata K545, the opening melody of the Allegro (one of the simplest and most popular melodies of the period) runs 1-3-5-7-8-9-8.
By the definition given in The Complete Musician, would the jump from the high 5 (G) down to the 7 (B) be considered a melodic dissonance, at it forms a minor 6th? This seems crazy, as the melody sounds extremely natural to me (and, I imagine, everybody else).