My high level wizard kicked Spiderman’s ass. It is not Superman but still a superhero so I think it counts.On your topic, the superhero question:
Are D&D characters powerful? Yep compared to e.g. DSA characters they are (most of the times).
Their magic capabilities are very high on higher XP levels.
Are they superheroes though? Well, let us see:
- Fly at will, through outer space if needed? No, unless you are of a winged race, and then it is only maneuverability class B
- X-ray vision or the like? Nope. You got some divination skills though for certain caster classes.
- Being really inhumane strong, like able to lift and move a whole truck? No. They are far stronger than most world elite professional athlets, but they are not "Superman"
- Invulnerable? No, though they can take in their package of hurt.
So they are above the things that normal IRL people could accomplish, and above many characters in other - more low magic, realistic and gritty - systems. But they are not "marvel-class" beings.
Are they heroes then? Hm, I would say that is dependent on their deeds also isn't it? If they are evil aligned and out for no good they could be villains as well , no?