The breakdown,
Some say yes, others no, though i agree with both sides on different topics, I do create custom thumbnails for most, Why? because typically when those see these thumbnails they tend to look at it in a way that suggests how much time you put into your videos. Say something like, man this looks nice i bet the video looks nice too! let me watch. On the other hand most of the time it doesn't matter, simply posting a topic someone finds interesting will cause most to click on the video despite the thumbnail, unless of course your are a click bait advertiser such as failarmy or the like, posting near nude females for the sake of clickage.
However if you cannot make, or simply refuse to, its your choice. You will gather a following anyways. And unless requested from dedicated subs.