I kind of see this both ways.
I still consider myself to be a beginner, but when I first started out I learned basic HTML and CSS and then learned Bootstrap. I liked Bootstrap because it allowed me to create some cool looking pages right away without being an expert in HTML or CSS. So in that sense, it was great as a beginner to be making functional websites that I knew would work across different browsers. I felt like I was accomplishing a lot in a relatively small amount of time.
Of course, the other side is that I eventually ended up in a place where I realized I actually knew very little about CSS and even less about responsive design. This led me to start digging into how Bootstrap accomplished certain tasks, which led me to learning a great deal about CSS. Since then, I have gone on to learn a great deal about CSS in general, and I no longer use Bootstrap in my FCC projects, just because I want the experience of developing without it.
Overall one could probably present good reasons to start with Bootstrap, and good reasons to ignore Bootstrap until you’re more comfortable with HTML/CSS.