Like people have said, it's very important, arguably even more than your actual content. I know Youtubers who even with 50,000+ subs already and 300+ videos, changed all their thumbnails and saw a massive boost. But another thing I'll add is to compare your thumbnails specifically to your Niche and competition. Create thumbnails that stick out compared to theirs. For me one of my competitions are motivation videos and they mostly have dark themed pictures of celebrities or athletes. My thumbnails are brighter and have whiteboard animation style and sticks out from theirs. Good thumbnails have made a big difference for me.
I also have asked friends who don't know my channel yet to search something on Youtube, and without knowing which channel is mine, get them to tell me which one's they'd consider clicking first.