Personally, I think that Bootstrap interface is good enough, but its widespread adoption lead to the perception I mentioned above.
Personally, an end-user has never asked me to make their software/web-admin look unique. A portfolio site? Sure, but an admin page? Not other than their official color/logo in the header.
Uniqueness is just not a concern with "normal" people, and with good reason as I mentioned above. Still, congrats on the nice work on the site/post.
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.
Sigh... bootstrap didn't get where it is by looking unprofessional.
I also don't understand wanting every admin interface one comes across to be a special snowflake, the opposite of what should be the case. We've known for at least 30 years (Apple HIG) that a standardized look and feel is a benefit to users, not a detriment. Not to mention dev maintainability.
I suppose billable hours decrease when reinventing the wheel is no longer necessary.