Don't get us wrong: there are some surprisingly serious plot elements in this book, such as Howl's terrible relationship with his sister (okay, not as bad as most of the sibling relationships in A Game of Thrones of course, but still—pretty bad) and Sophie's extremely poor self-esteem.
However, while there might be events in the novel that strike us as kind of grim, the overall tone of the book itself is light and funny. There's a ton of dialogue, a lot of which is teasing and sarcastic, and the passages of description tend to be action-heavy instead of focusing on lots of description.
Let's take a look at a passage: this scene takes place when the scarecrow arrives at the door of the castle while Michael and Howl are both there to see it. Howl holds the door against the scarecrow and then this:
"So you won't go?" Howl said. And the turnip head slowly moved from side to side. No. "I'm afraid you'll have to," Howl said. "You scare Sophie, and there's no knowing what she'll do when she's scared. Come to think of it, you scare me too." Howl's arms moved, heavily, as if he was lifting a large weight, until they were raised high above his head. He shouted out a strange word, which was half hidden in a crack of sudden thunder. And the scarecrow went soaring away. (8.6)
Even though Howl actually admits that he's scared (which basically never happens), he still takes the opportunity to joke that "there's no knowing what [Sophie]'ll do when she's scared." Once Howl has had his little joke at both the scarecrow and Sophie's expense, he blasts the scarecrow away with magic. The paragraph uses simple, descriptive language to make sure that it's clear exactly what Howl is doing in the scene.
Howl's joking tone with the scarecrow and his clear, easy-to-imagine magic gestures make this passage seem both light-hearted and action-packed, which we think is pretty consistent for the tone of the whole book.