Of course it's important. Your channel is your brand, and you got to sell your brand. People are instinctively drawn to images and graphics. The more professional your channel looks, the more professional feeling it will get. When you have a professional looking banner, avatar, thumbnails, etc. people are more willing to click on them and/or subscribe. Most people do this without realizing it.
Let's say you have two websites. Both websites have the same information, but one has graphics and images, while the other one doesn't. They are essentially the same websites, but people will enjoy/keep coming back to the website that has graphics/images.