I think if you were to tell someone that they have a "sweet mouth" they would take it as a way of saying their mouth is physically attractive like "you have pretty lips". If you want to acknowledge that someone always has a kind word to say, then simply being literal and saying, "You're very nice. You always have a kind word to say." or "You're very kind hearted." should work.

Hi Haru, I would not use neither Sweet Mouth or Sweet Mouse. People will not understand you. It is not negative, just, nobody ever says it.

This is a helpful question, Haru. :)

smooth tongued Synonyms and related words: Ciceronian, Demosthenian, Demosthenic, Tullian, adulatory, articulate, bland, blandishing, blarneying, buttery, cajoling, complimentary, courtierly, courtly, disarming, eloquent, facund, fair-spoken, fawning, felicitous, fine-spoken, flattering, fulsome, glib, gushing, honey-mouthed, honey-tongued, honeyed, ingratiating, insincere, insinuating, mealymouthed, obsequious, oily, oily-tongued, silver, silver-tongued, slick, slimy, slobbery, smarmy, smooth, smooth-spoken, smug, soapy, soft-soaping, soft-spoken, spellbinding, suave, suave-spoken, sycophantic, unctuous, well-spoken, wheedling