In my own point of view it is important, it is one way to increase it's readability, uniqueness and individuality of your blog. Our blog design most often describes what characteristic we have, Happy, Free spirited, moody, black etc...Improving your blog design is part of becoming a successful blogger.
How to improve your blog design?
Use relevant, unique and interesting picture in your blog header and in your post - With a little bit dedication you can design your own header using all the tools that are free such as paint, picnik.com, imageshack.com and more. Let go of that artistic side of yours (You thought you don't have, Your wrong! You have something that is hidden and waiting to be discover). I will tell you the secret..shhhh (That header that i have, i made it using paint and picnik).
Make your font size readable - I remember my brothers blog he's background is black, the font are too small...he gets people commenting about the font. Use at least 11 pt. size for your body text.
Match your Body text and Background - Again, going back to my example black background, small size font is a no no, then come the color of the text -- If you want dark background, use white text or the other way around. It is more easier to use white background with black text because it's easy for the eyes to read.
Make Over your blog and you will see the response. Make it Unique, interesting and then you will see that blog design is important for your success.