Okay but it's all good saying it doesn't look good but then how can improvements be made cause what you've said is extremely generic and doesn't provide a way to make it better.
I would make the Text Bigger or all in Caps so it stands out more and give it a color that POPs. Also try using Complementary Colors (Google Complementary Color Wheel) which can be very helpful when trying to make your Thumbnail stand out more.
Also make sure your text on your Thumb is complementing what your title is, which is why I suggest adding your title to go along with the Thumbnail.
Remember, a lot of people are looking at Thumbnails on their phones, so what looks good big may not be so clear when it is shrunken down.
The problem I have when asking about a Thumbnail is many people whom are not gamers do not get the "Thumbnail". Along with the Thumbnail, put the title of the what your video is going to have (Not on the Thumbnail) but as another source of reference. It's kind of a double edge sword when asking about a Thumbnail if it not within a well known genre.
I have had a few channel reviews and they look at the thumbnail, which is a screenshot from within the game I play and in reference to the video. Yet because they don't know the game they "Don't Get" the thumbnail. If I was playing Fortnight or PUBG then they know it because those are over-popular games that everyone knows or have seen at one point. Now if it was a cooking channel showing food, a tech review showing a phone, or something like that, them most will get what they are being shown. Hope this makes sense.
Also ignore comments like the one above. If they offer no reason or other helpful information, then it's no information at all that they are providing.
