Green’s Dictionary of Slang

(US) sentimental.

1965		(con. 1920s) Carmichael & Longstreet Sometimes I Wonder 104: They like marshmallow music.
1999		Guardian Rev. 15 Oct. 22: The plot is sheer marshmallow. Its attractive but uncharismatic leads and leaden script make it seem a bit like an early 60s Elvis movie.