As a long-time blogger, I can say from experience: It's much harder to get any traction with an audience if you don't have a specific topic or theme. But it depends on what your goals for the blog are. If your primary aim is to express yourself, and to have a place to keep your writing skills sharp, then it's okay for your blog to reflect that. But if you're focused on building an audience, your best bet is to find an under-served niche and fill it. It's like choosing a restaurant for dinner. Some restaurants thrive on doing well at a wide variety of cuisines. But most restaurants specialize. People come to them when they want something specific.
So, to summarize, one approach to blogging is more personal and expressive, the other is more business-minded and goal-oriented. If you just want a personal blog, there's no right or wrong answer as to how to set it up. If your goal is to build an audience, then you should do market research, and find out what people want that they aren't getting.
I think most of us, myself included, get in trouble when we think one blog is going to excel at BOTH aims, personal expression and audience-building. I won't say it never happens, but you have to an an extraordinarily compelling voice if you're going to attract people to sign on to your own meandering personal journey.