The Collaborative International Dictionary of English v.0.48:

Mellowness \Mel"low*ness\, n.
   Quality or state of being mellow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mellowness
    n 1: kindheartedness through maturity or old age
    2: a taste (especially of fruit) that is ripe and of full flavor
    3: the property of a sensation that is rich and pleasing; "the
       music had a fullness that echoed through the hall"; "the
       cheap wine had no body, no mellowness"; "he was well aware of
       the richness of his own appearance" [syn: fullness,
       mellowness, richness]
    4: a soft shade of a color; "a mellowness of light and shade not
       attainable in marble"
    5: geniality, as through the effects of alcohol or marijuana

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "mellowness":
   affability, agreeability, agreeableness, amenity, amiability,
   amicability, bassness, bliss, blissfulness, compatibility,
   complaisance, congeniality, cordiality, deepness, delicacy,
   downiness, easiness, enjoyableness, featheriness, felicitousness,
   flossiness, fluffiness, fullness, geniality, gentleness,
   goodliness, goodness, graciousness, harmoniousness, hollowness,
   insolidity, leniency, lowness, mellifluousness, morbidezza,
   niceness, nonresistiveness, nonrigidity, plangency, pleasance,
   pleasantness, pleasantry, pleasingness, pleasurability,
   pleasurableness, pleasure, pleasurefulness, plushiness, pulpiness,
   rapport, resonance, richness, satininess, silkiness, softness,
   sonority, sonorousness, sponginess, sweetness, tenderness,
   unsolidity, velvetiness, vibrancy, welcomeness