If your focus is growing your channel, then you should consider focusing on a single niche. If your focus is to have fun on YouTube and not so much on growth, then it's fine if you have multiple niche (just know that you won't grow as fast).
I assume you're talking about the Bulk Thumbnail Overlay? If so, this bulk feature is good for if you need to add in overlays to your thumbnails.
For example, you have a brand logo that you want to add to your thumbnails, but you don't want to reupload the thumbnails for each one.