Q: Would it be OK to name an App using a trademarked word in the name? For example, BuJo Layouts.

A: This is a "fair use" question. Your goal is to let your customers know that your product is intended for use with Bullet Journal. Conveying that message in the app name would be an obvious benefit to you. The problem is that the holder of the Bullet Journal and BuJo trademarks might think that your app name would create confusion, with potential customers thinking that your app is made by them, or sponsored by them, or otherwise related to them. They don't want that kind of confusion to occur, because if something goes wrong with your app or it doesn't work, customers will blame that on them.

That, then is the issue--your interests and their interests conflict. The conflict gets resolved by analyzing whether the confusion that they want to avoid is "reasonably likely" to occur. This analysis hinges on a number of factors, and there are exceptions for certain kinds of fair use of the trademarks of another. For example, if you are providing a review or critique of their product, then you likely qualify for a fair use exception. Here, while the facts provided are limited, I would suppose that your app is meant to complement or work with their product, and if so, it's fairly likely that the reasonable consumer might be confused that there is some form of relationship/sponsorship between the trademark holder and you. That would, most likely, mean that your use would be problematic.