Synonyms
ding-dong
(with) a sound of bells
fickle, irresponsible
frivolous
not serious or important; enjoying pleasure (too much)
trivial, silly
scatty
behaving as if a little mad
giddy
silly
not sensible; foolish
absurd, giddy, foolish
FEATHERBRAINED as in Wiktionary
FEATHERBRAINED as in Wikipedia