To me, thumbnails serve different purposes for different situations. The 2 situations for me are looking for a new video to watch and looking for a specific video I've watched before.
If I'm looking for a new video to watch, I'll primarily look at thumbnails to give me a sneak peak at what I'll see when watching it, kind of like looking at screenshots on the back of movies or video games. If the thumbnail doesn't show anything from the video and instead is a completely fabricated one, such as just someone's logo and some words or some other picture that doesn't appear anywhere in the video, I'll be slightly put-off, but if it looks pretty decent otherwise, I'll still watch it if the title and subject of it sounds interesting.
I'm I'm looking for a specific video I've watched before, I'm mainly looking at the thumbnails to recognize the video.
For my channel, I dislike making thumbnails. They are just another tedious step to me, like thinking of descriptions. I like the video-making part, but I wish everything else was automatic. I still make thumbnails for every video, though, because I think it is important. But to minimize the time and work needed to do it, I follow a consistent format with each one. I use the TubeBuggy Chrome extension to generate a thumbnail after the video is finished processing. I pick a screenshot that has the clearest and creepiest face I can find, zoom in or out to make it full frame, add a red border and my logo in the upper left corner, as you can see in my video in my signature.