I understand this and need your advices. 
The existing bootswatch webjar is directly bundling all the git repository archive provided from a given tag (http://www.webjars.org/listfiles/bootswatch/3.0.3+1). 
It is including all themes CSS (and LESS) files (ex: https://github.com/thomaspark/bootswatch/tree/v3.0.3%2B1/spacelab) and various bower dependencies like bootstrap, fontawesome ... (https://github.com/thomaspark/bootswatch/tree/v3.0.3%2B1/bower_components)
But there aren't really usable because the provided CSS are supposed to replace the ones inside a bootstrap distribution. For example, in CSS you'll find relative paths to ../fonts/ to access to bootstrap icons while they aren't provided in the distribution archive.
It is also including various useless resources for us like bootswatch website pages/resources and all of that is creating a very large archive (and sadly useless) webjar.
I can avoid to take bootstrap files from the distribution and instead add a bootstrap webjar dependency to get these files. But this dependency will be used only at the bootswatch packaging time. I'll probably propose to put it with a provided scope as the webjar user won't be able to control the bootstrap version included in the bootswatch webjar. Does it make sense ? Is it cleaner ?