That's not true. It's a common misconception used to discredit DC characters. There movies could be better written but the actual characters on written page are pretty similar.
For decades, Marvel characters were supposed to be relateable everyman characters while DC was full of billionaires and gods and aliens with special abilities. Tony Stark? Thor? Silver Surfer?
Marvel Characters were written with real world problems but DC characters have been written the same since at least the 70s. Hell, half the time the people who created the characters or wrote for Marvel also wrote for DC! The only major difference is that a lot (not all of the time) of the time, DC characters are loved by the public while Marvel characters are feared. And even then there's tons of examples that contradict even that.
I think thats something worth having a conversation about. I personally share the opinion that DC's characters have a harder time being "cool". Not so much that "they cannot be cooler" (or whichever adjective you like), because they can - in theory - and as you have pointed before, its all about execution, but thats the issue IMO. Its much harder for Warner than it is to Marvel. Marvel has an edge in that several of their comics were not widely known/popular and they took several creative freedoms to make them have an impact when they hit the cinemas. Guardians of the Galaxy for example - A huge amount of the movie adaptation does not reflect the comic counterparts, from character portrayals, to backstories, a lot of the source material went under the bus. But the movie is fantastic, fun, commercially succesful and they became very popular overnight. No one cared because these characters were very much unknown. Even the fact that the movie was announced felt weird when its was originally announced. Those creative freedoms are harder for DC properties because their characters are far much popular, and I do believe fan backlash can be a reason for their more rigid portrayals.
The other thing is that Warner really struggles with how they adapt these characters. The entire Suicide Squad film was desperate to look "cool and edgy" and I didnt mind Aquaman that much, for a while he has been a cooler character that people give him credit for, but his portrayal as "the beach bro" really didnt hit well with me.