I am using the singular form of the categories for my niche site.
"Oven" not "ovens."
"Blender" not "blenders."
Which is more suitable?
Unless you look at trend reports in your SEO software and see that for some reason, either the plural or the singular is getting more traction, specifically in your niche, it won't mater for SEO if you use one over the other.
Search engines are generally smart enough today to know that "oven" and "ovens" are basically the same query. So the best thing you can do is decide what makes more sense for your blog or website, and stick with one thing.
Keep in mind that for your content to show up in various rich results, you will still need to implement proper structured data on your category pages; this will play more of a role in your SEO.
If your blog or website sells home goods, I believe it may be more appropriate to use the plural. (You're selling ovens, not a single oven.) But if you're not an e-commerce operation and just have a blog, it may not matter as much. (Personally, I'd still go with plural, but again, your niche, your purpose, and your reader preferences may play a role in your decision.)