Readers like being able to categorise the website they are on. For example, a reader looking for information on historical vehicles will most likely want to visit a website that focuses on historical vehicles. If they arrive on a website that focuses on cars in general, they will also find this acceptable. However, a reader would probably question the veracity of the information about historical vehicles on a website that reviews cookie dough recipes.
Now, there are some niches where you can focus on multiple niches with one website, but these niches are usually related to each other in some way. A good example of a website that focuses on multiple niches would be a health and beauty website. The health and beauty website could focus on weight loss, make-up, orthodontics, and many other related niches without seeming unreliable and untrustworthy to their visitors.
If you're going to focus on multiple niches with one blog, then make sure the niches are related.