No, you wouldn't be classified as a bad developer if you do use frameworks.
The developers behind those famous frameworks had a goal to make developing website more efficient and fast. They have this mindset to solve some of the problems do web developers experienced when creating websites. Also, those who created it are not some kind of newbies in web development. They are experienced web developers as well who throughout the years that already have classified some of those problems and tried their best to address and solve it.
Using a framework is a personal preference however. Not any one will love it. Others will hate it.
But if you want to use it and in order to utilize those frameworks at its best, you have to learn or know the basics of it - which means learn the foundation language behind it which is CSS.