This is going to sound %100 like sour grapes, but here goes:
I hate it when I click on a comic with an amazing front cover, but all the pages are half-assed. Like you can tell they're an amazing artist from the cover illustration, but when you look at their pages, it looks like they pulled it out of their ass in 30 minutes in comparison! I'm sorry, but if you have an amazing cover artwork, the content of your comic should reflect the same amount of effort on some level. What usually ticks me off is when I see this in popular comics, staff picks, and even front pages features sometimes. I'm assuming they rush through the art so they can get more updates right? That's all well and good, but I will always be a firm believer in quality over quantity.
Also, I REALLY fucking hate it when people get a separate artist to do the cover artwork for their comic! You're fooling the reader just to get your comic purchased, and that's not okay.
I bought an indie comic at a comicon last year, the cover looked really professional, but the pages were incredibly amateur. I thought maybe the artist really improved later on or something, but when I checked, they had actually hired a professional to do the cover. Do the cover art yourself. If you don't think your artwork is good enough to attract people, then maybe you shouldn't be trying to sell you comic yet? Put it off and practice more. If it's your comic, whatever's on the cover should be made by YOU.