Some are, some aren't. Certainly he was never a pure escapist character like Captain Marvel, however. But by escapism I mean as an escape from the depressing realities of real life, an escape into imaginative and more hopeful worlds. That's also why the post-apocalyptic subgenre has never been my thing, although I do love Planet of the Apes and Escape From New York.
Certainly I like for fantasy stories to relate to the human condition to some degree, but I'm not a fan of the gritty realistic sort of stuff.
Like, lemme break it down with my personal favorite Superman stories:
Kingdom Come: You've got themes of vast power being wielded in ideological clashes, you have great people trapped in no-win scenarios, there's mass murder, there's dead Lois, there's the burden of moral responsibility and the tole killing takes on a person's soul. Plus all the religious iconography and themes.
For All Seasons: Superman fails to save a woman's life during a crisis early on in his career and has to deal with the fact that he can't save everyone for the first time. Deals with how different people perceive great men and how those great men have to shoulder the burden of those perceptions as well as the theme of how the never ending battle truly is a never ending battle, and we don't fight it because we think we'll ever win but because there are things worth fighting for.
Whatever Happened to the Man of Tomorrow?: A bunch of supporting characters die, Mxyzptlk turns into a Lovecraft monster, all in all this comic is dark. Like all great last stand stories, it's about a heroic person facing death with dignity, and like many of the greats contains something of a bittersweet happy ending that he hero fought like hell to achieve with the hope of some kind of future rebirth.
For The Man Who Has Everything: A story of temptation, desire, and regret that explores the responsibility Superman bears and the many facets to his personality.
DC Comics Presents: Superman and Swamp Thing: Superman has an identity crisis as he finds that he's dying of a terminal illness.
What's So Funny About Truth, Justice, and the American Way?: A moral conflict between Superman's ideology and that of a group of violent upstart superheroes in which he stands by his principals despite them being more difficult than the looser morals of his younger counterparts.
All Star Superman: Superman is dying of a terminal illness. Again. Explores Lex Luthor's twisted psychology, how Superman dealt with his father's death, how he faces his own death, and brings an enormous amount of pathos to the colorful cast of the Superman comics.
The Death of Superman Trilogy: More dead Superman (although this is the only one where he actually dies). Shows him making a heroic last stand, follows his friends and family through the grieving process, and shows him reclaim his name and life largely through force of will.
Lex Luthor: Man of Steel: Delves deep into the mind of a murderous megalomaniac and how he rationalizes every awful thing he does as somehow being for the betterment of society.
Crisis on Infinite Earths: Supergirl dies. A bunch of characters die. It's not just a Superman story but he's at the heart of it and he gets some of the best moments.
Red Son: Explores how the cast of Superman would have developed under different circumstances. Superman is the villain, raised to believe in a tyrannical system, and Lex Luthor is the hero, a patriot sworn to defeat Superman, but they otherwise keep all of their most noteworthy character traits, such as Superman's selflessness and Lex's massive ego, making it a much more morally gray story with both sides of the conflict having heroic and villainous traits. All in all a character study of both of them under different ethical circumstances.
John Byrne's "Generations" trilogy: A ton of characters Superman is close to die while he has to carry on into the future, as well as addressing the politics of the eras the story passes through.
Superman: Secret Identity: Themes of identity, isolation, love, responsibility, and looking for purpose in life.
I wouldn't label any of these as escapist. It's a semantic thing, but what you call escapism I don't think I call escapism. Not really, anyway. "Not being dark" and escapism aren't the same thing. Something can be light and fun and still be about something meaningful. True escapism isn't about anything meaningful, it just insulates people from the world.
Even then, though, I disagree with the idea of escaping from the depressing realities of life. I don't think literature's goal should ever be to enable people to escape from the depressing realities of life. It should be to help then develop the tools to
deal with the depressing realities of life.