It’s very, very important for fiction to have an appropriate tone. As for the difficulty of that, it’s strange. On one hand, I’ve seen many otherwise bland stories at least manage to keep an appropriate and decent tone. On the other, I’ve seen just as many ones-and these include professional and/or long works- that don’t.
I view maintaining a suitable tone as a kind of art that’s not easily explainable in terms of how to do, because you have no one answer for every story. What I also see is that getting the right tone is a very key factor in separating a proper story from a bowl-of-ingredients clunker where you have events and characters but they’re not really integrated. Not everything, but a big part.