The fact that Bootstrap is widespread, and therefore perceived by some unprofessional, does not make a user using it unprofessional, nor does it make the framework unprofessional.
Are Django or Flask unprofessional since they are widespread?
Are Django or Flask unprofessional since they are widespread?
Bootstrap is good and clean framework, it is _great_ for developers. But just because it is so widespread and highly visible, there's some of prejudice around it.
Nonetheless, I'm using Bootstrap in my projects and happy with it.
I'm happy to use it too with very minimal tweaking because it looks good out of the box, way more pleasant than browser defaults, it works well, fairly well maintained.