Do I think that better reach could have an impact on donations? Sure.

Do I think that lack of marketing is the reason for FOSS donations lagging behind other donation causes? Not at all; I think they are actually losing out on impacts, in most cases.

FOSS project donations are usually done by people who use the tool, and are interested in seeing it get improved. It's not a "good cause" donation, like feeding kids. If you are collecting money to help people, donors don't expect to receive something in return for giving. But I think it's incredibly unrealistic to think that people will see someone building a software tool, not have interest in using it themselves, but still donate money to support the project anyways.

Marketing a tool that isn't garnering much interest already probably isn't going to see the tool get much additional uptake, especially with how much free marketing already exists in the FOSS space. If you post your software on Reddit and Hackernews and ArsTechnica (all free to do) and aren't seeing interest, you're probably not going to be massively helped by a marketing org stepping in.