Here's what a serious GUI framework must have:
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Good enough Unicode rendering
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Accessibility handles
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Scaling support
Lacking anything of that, it's not serious. Note how scriptability, crossplatformity, customization, free license, uniformity, and cetera are not included. They're all probably easier than Unicode rendering though.
Not being serious by my definition is not bad. It just means I think it's a bad idea to develop an application with such a toolkit. Some visual games might be fine.