I take your point, but don't all performers need a big ego to do what they do?
Agreed Dietrich didn't have the greatest voice but she had what Garland & others with better voices didn"t have-charisma. She could stand in front of a microphone & with a shrug of a shoulder or a raised eyebrow could convey a lyric so clearly & succinctly without needing expansive gestures.
She also had longevity & was entertaining in packed auditoria long after many others had retired or succumbed to the ravages of drugs or alcohol. She was no angel but had discipline & a strong work ethic & although she loved to party, work came first.