Yes, you can reduce the amount of brown sugar and it will have no significant effect on the recipe other than reduce the sweetness. I did try it without the brown sugar and while it works, I much prefer it with the added sugar. You can also reduce the amount to a few tablespoons instead of omitting it completely if preferred. I also don't recommend reducing the amount of maple as it's important to the texture of the granola. Otherwise, you'll get very crumbly granola with no clumps.