This depends on what kind of website/blog you start. If it is just a personal blog, then yeah putting a bunch of ideas in one place is okay. If it is a blog or website that is meant to be helpful for others, like on writing or on a subject or issue, it is best to keep it about one subject. The reason for this is search engine placement. When you have a lot of posts about one subject Google and other search engines begin to direct more users to it for that subject.
So if you are going to start a blog about your life, it's fine to have a lot of post on many subjects. If you are going to start a blog about helping writers or about video games or whatever, if you want those people to be able to find your blog, it is a good idea to keep in on one subject. It wouldn't be a hard and fast rule, of course, sometimes the posts could be "off subject."