It's easy to cherry pick pieces to prove your argument either way. Holding up any two pages for comparison is pointless. Even individual artists have pages that are outliers in both directions. Your question was In General, Has Comics Art Improved over the Years? I would have to say yes.
For one thing, today's art process has greatly improved. The digital inks and colors make even mediocre line work pop; they make good lines jump off the page, and great lines turn into instant classics. Indie comics that don't adhere to a house style look so much better now. They used to all look like very cheap xerox pages of pencil drawings, and the equivalent stuff has a professional quality now.
When I look over the rack, I see a lot of good, bold and interesting art grabbing my eye, more than the old days. Is there crap, too? Absofuckinglutely. But OP asked about overall, not specific, so I'll say yes.
Now, the other questions that people are addressing are composition and storytelling, especially of covers. These have definitely gone down. Covers are made to be posters, now. They have less to do with the interior story than ever before. Editors and artists freely admit that they often create covers months in advance without ever seeing the script or knowing the story. (I rarely vote for pin-up covers in Cover of the Week because I hate this trend.) Ironically, the artwork and creativity on covers like this can be wonderful. Frex, the Fearless Defenders covers blew me away! And yet, I never read a single issue. No desire to crack it open at all. But damn those covers were great!