I always learnt to keep markup and styles separate as much as possible, because that was beneficial for re-use, maintainability, etc. I would see the JS framework as providing the markup part, so surely the styles should be an orthogonal concern? Would this library not be better as a CSS library that apps can use regardless of the JS (or lack of JS) that they choose to have?
I find myself taking a hybrid approach these days: common styles and variables (primarily variables) in global SCSS/Stylus stylesheets, and component-specific styles defined in the respective Vue components.
As it turns out, I have very few truly common CSS classes.
EDIT: To address the re-usability aspect as well: this approach absolutely makes it more difficult to re-use styles across other frameworks; but if I'm moving from Vue to some-new-framework, stylesheets are the least of my concerns.
Bulma -> Buefy Bootstrap -> VueBootstrap, et al.
That said, if you're already using Vue, then a Vue-based framework is more convenient.
But how cool is that? A css file. Not as cool and professional as a 'Design System for <frameworkname>'.
Also, in my experience is that once a developer has mastered a framework, they are so proud of it the tie everything to the framework. To pledge their allegiance. And to signal to the pack "Hey, I'm part of the family!".
Why do you feel the need to make sweeping generalizations about the author and the author's work? Just shitting on someone/something for the sake of it?