Have you talked to a lawyer regarding possible IP Problems?
They may have a problem with the use of "Marvel" in the domain name though.
"You may not create derivatives of Marvel Property, or use the 'Marvel' name or any other Marvel Marks in the title of your App without Marvel's prior written approval."
They might let you if you ask, though. It's not like it's a commercial app.
I think it's good that you added links back to the Marvel website, and attributed the data to Marvel, following their guidelines: http://developer.marvel.com/documentation/attribution
Some developers overlook the boring API documentation and rules.
That reception is probably not uniform among Marvel, especially since they have to mount an affirmative defense against trademark infringement. But just don't get too popular and it will never matter. ;)