Yes. One topic signals to the search engines you’re an expert in what you’re doing. But this is something your readers perceive as well. And having multiple topics seems unprofessional.
Just think about it. Let’s say you encounter a site on dog toys. And there are bunch of articles, all related to this topic.
And then you find some article there covering top beard products.
How likely would you trust buying your morning hygiene products from a dog toy site?
Probably not very likely. Because it just seems off.
Even though this might be an extreme example, your visitors and potential customers feel some level of uneasiness around multi-topic blog.
Besides…
If you want to increase your rating, best thing you can do is to find very good keywords and write decent content.
For example, if you’re interested in finding keywords that even the professional tools cannot find, then read more in this SEO book and find these golden SEO nuggets.
Senior Software Developer with experience in Machine Learning, Artificial Intelligence and search engine algorithms.