In my opinion, No.
Instead of stating it as a bad developer. I would say if you started with basic CSS and directly jumped in some frameworks you would be missing out all the qwerky things you need to sometime/could do with vanilla CSS.
Getting layouts done using float/position only, a tooltip, a user-friendly modal/popups without flex-box or grid would be one of those situations. So if you are good at the basics and now get into the frameworks world, no problems with it. Infact you will have to when your product gets bigger. If you are building your own, lets say framework for your product. It will take lots of time/bugs to get it to the state that a framework already is in. Why not focus on your product/business logic instead right?.
But if you are working on a tiny product then maybe you can use your vanilla css skills with SASS to boost productivity. There would be no need of frameworks. If you are using some framework which has hell lots of utilities/features/widgets and stuff for building a todo app wouldnt make sense. Instead if you want the exact same designs then dig into the codebase and copy just thats needed (and credit them).
In the end it comes to personal choice sometimes, becuase you might not like few things about a framework maybe the components/code/structure. I personally start from scratch/use my previous css utilities that I treat as a base for any of the projects that I am working on.