Not at all. TubeBuddy will show your variations to a small percentage of your potential viewers (around 50%). When it comes to thumbnails, if a variation is worse than the control (the original thumbnail), fewer people will click on it.
However, metadata tests are a bit more risky as changes to your title, description or tags will cause your video to temporarily drop out of search results while YouTube reindexes it. This can take between a few minutes to a few hours.
In other words, A/B test YouTube thumbnails frequently, A/B test YouTube metadata (title, description, tags, etc) intentionally.