Marvel Comics hated his casting at the time. Just like when Heath Ledger was cast as the Joker in Christopher Nolan's The Dark Knight, Keaton's casting was hit with a lot of backlash at the time. It's a common occurrence for some fans to get upset when popular figures like Batman are reinterpreted in new adaptations. More recently, Robert Pattinson faced backlash when being announced as the new Batman in Matt Reeves' iteration, due to many despising the Twilight saga. It's an ongoing phenomena that will never not be a thing.
Marvel and DC Comics have crossed over many times throughout the years, but there has always been a not-so-subtle rivalry between the two companies. This applies to all Marvel and DC properties, no matter the medium. Naturally, creatives on both sides have been asked for their opinions on their competitions choices. For instance, Marvel legend Stan Lee was quoted as saying he doesn't like the idea of Superman but prefers the idea of Batman, a more down-to-Earth hero.
In 1989's Incredible Hulk #355, Michael Keaton's casting as Batman in Tim Burton's film is debated by two characters clearly modeled after movie critics Gene Siskel and Roget Ebert. The Siskel analog outright states Keaton's casting is "simply inappropriate." Although the issue tries to back up its claim with a counterpoint from "Ebert," it's more than obvious that this is a direct criticism aimed at DC and Burton for making the choice. Even the counter-argument describes Keaton as "off-beat" which doesn't come across as being that accepting of the premise of this new Batman. The choice to allow such comments in a mainstream issue of Marvel Comics is an obvious ploy to rock the boat in the superhero world, essentially stating that Marvel would never do such a thing.
Also, the mention of "the camp 60's" is another shot at DC and its live-action interpretations. It's a clear reference to Adam West's Batman, who was definitely less serious and more on the comedic side ,but is still a beloved version of the Dark Knight. There is nothing wrong with having superheroes more on the "camp" side, but clearly characters in the Marvel universe believed otherwise. Additionally, the assumption that Keaton wouldn't be taken seriously was based on his largely comedy-driven film repertoire including films like Mr. Mom. It was believed that casting an actor with comic chops was bound to doom any chance of a serious, dark adaptation that would be faithful to the comics.
Perhaps die-hard fans have gone back and read issues like 1989's Incredible Hulk #355 to dig into the never-ending rivalry between Marvel Comics and DC Comics, however, for the most part, fans are probably only aware of the big, public outbursts of backlash throughout comic book history. But small gems like this issue's blatant criticism of Michael Keaton's casting as Batman are fun finds when trying to build the bigger picture surrounding their infamous rivalry.
