"Superman" hasn't even opened -- and it's taking a beating in conservative media over possible pro-immigration messages due to comments from the director ... and one former Superman says that super-sized mistake might hurt the box office.

Here's the deal ... James Gunn told The Sunday Times the movie is about an "immigrant" and "basic human kindness" -- a value he says we've lost as a country. Those remarks were pounced on by conservatives as a clear indicator the flick will be "woke" and preachy amid ICE raids via the Trump administration's relentless deportation efforts.

Dean Cain, who wore the red cape in "Lois & Clark: The New Adventures of Superman," tells TMZ ... he was so excited for the new movie ... until Gunn had to break out the political Kryptonite -- and sour a portion of the fandom.

The outspoken conservative actor and occasional political commentor is pretty sure the immigrant remark controversy is enough to ding the box office returns ... but he still hopes the movie is a success. Of course, Fox News’ personalities railing against the film won't help ... such as Kellyanne Conway saying, “We don’t go to the movie theater to be lectured to" and Jesse Watters joking that "MS-13" is written on Superman's cape.

DC tells us is so frustrating as the comments from Gunn weren't even necessary ... 'cause we KNOW Superman is an immigrant -- he's a freaking alien! What's more, Superman was created in 1938 just prior to WWII breaking out ... and "Truth, Justice and the American Way" were -- and still should be -- crystal clear values.

Dean knows everyone has an opinion ... and the country's temperature has been cranked to high the past few years ... but he tells us not every piece of art needs a message! Some things can just be fun ... and he's holding out hope "Superman" is a hell of a lot of fun.