Having silly stuff doesn't make the game lighthearted. Metroid is pretty serious, but it's not dark and depressing like Hollow Knight. I can't imagine The Knight's victory screen, for example - anything actually victorious would look remarkably out of character, and anything in-character would be really weird within such a flashy game. Whatever Hallownest stage we got would be filled with gloomy songs, the game barely has any upbeat tracks (though I can think of 3 or 4 that wouldn't be too bad), and the atmosphere, if recreated accurately, would be just sad.
At the same time, though, it'd be an awesome pick.
You needa check the definition of lighthearted then, silly things most certainly entertain and amuse. The Metroid games I've played (Super, Prime 3, Hunters) have no humor or charm in them whatsoever to lift spirits/lighten the mood, whereas Hollow Knight is full of it.