I think the reason why light novels have simpler words isn't for a "lighter story," but because the target audience has a more limited vocabulary. Just like most YALIT actually. Harry Potter is actually a really good example of this. Sure, you'll have books by Michael Crichton, father of modern storytelling, that can be understood by most despite being masterpieces. But it's actually a requirement for light novels. 

And a little more trivia, a visual novel without player interaction is called a kinetic novel.