Can a tone be too light?

This may belong in Basic Writing, but I wasn't sure as it's kind of an opinion based topic.

While writing a nonfiction book, while I've seen some books with a lighter tone (Stephen King's On Writing), is it okay to use the same tone during the proposal stage, or would that turn off a prospective agent/publisher?

If you've written a nonfiction did you use your own voice/tone, was it uber professional, or light. I find my proposal is riddled with my bitterness, anger, and bluntness.

When I say light, I don't mean humorous. More along the line of a conversational tone, like you were getting information from your best friend or sibling, but not so informal that you'd hear it in a bar...does that make sense?