It's not that John Byrne was a bad writer; on the contrary, the Man of Steel miniseries was quite good. even if it seems dated and Reaganesque today. But the changes he made to the Superman mythos stifled the character for years to come.
In the previous eras, Superman was fast enough to travel to other planets.
For the Golden age this was not true until after 1943 when Superman could fly and he did not get superspeed powerful enough to go to other worlds until 1947 by which timethe Golden age already in its decline.
Here are some examples of how SUperman suddenly became Stupidman in the Silver-Bronze ages. Luthor lost his hair after making an antidote to Kryptonite and yet when Superman fails to create an antidote he stupidly thinks that there is no antidote. Go back in time as a phantom and watch him make the formula. ARRGGH.
Another example, in one story Lois and Lana are exposed to the same disease that killed Superman's parents. Superman runs all over space and time looking for a cure but fails. He even excepts the BS
from his friends in the Legion about how they cannot change history even though alterante future were well known to Superman. Finally Stupidman realized he was exposed as well and he has the antibodies in his blood; Well DUH! that is middle school biology you Supermoron.
To be fair to Superman the villians tended to get written as idiots as well with the Challange of the Superfriends cartoon being a prime example. The first story showed promice but the consept quickly spirialed down to pathetic writing and even poorer logic.
For example in "Superfriends Rest in Peace" the Superfirend instead of destroying this crystal that can kill aa ofthem bury it beheath the Hall of Justice. Where the Legion of Doom can get to it. fter the LoD get the box and Luthor fails to open with his gadgets Sinestro proclaims that is must be made of a weak Kryton steal that only Superman can open. HELLO Legion of Idiots, you have a member just as strong as Superman - his name is Bizarro. When Luthor dresses up the box as a ray gun does Superman X-ray it to see what it really is? No he does juat at in the comic and act firsts and reallizes he royally screwed up. Then the LoD doesn't notice that in the battle with Sinestro Hal's green energy knocks Sinestro's beam away. ARGH the real Hal'sring cannot effect yellow in any way. Sinestro where is your brain? As a former GL you know this.
Then to finished up the stupidity after all this hassle do the Superfriends finally get the message and destory this thing? NO instead they have Apache Chief go down in a radiations suit, find the crystal shove it into a high-tech bazooka and shoot the thing in space where any freaking two-bit space hood could get the thing forcing the Superfriends to go
through this loopy plan again.
After this episode one could almost envision the following scene at the Hall of Doom (The Legion has escaped AGAIN).
Bizarro: Me make horrific discovery.
Luthor (sounding bored): What is it, Bizarro?
Bizarro: Me discover that me bring back virus that turn people into idiots.
Isn't it wonderful Legion of Doom now dumbest people on planet, BAWHAHA.
Luthor (putting head in hands): "Now its clear why we can't defeat the Superfirends. We even I the great Lex Luthor now have the collective IQ of a rock."
Bizarro: Me also have wonderous news. Superfriends also infected with virus. WAHAHA. Why can't only Bizarro and Legion of Doom friends be dumbest people in world.
Luthor (looking at the wailing Bizarro starts looking for something in his
podium): Here Bizarro I have a little something for you. (fires an energy
weapon).
Bizarro: "Luthor must really be Bizarro's friend, he try to kill me with blue
Kryptonite (falls over out cold)."
Luthor (throwing his ray gun over his shoulder): Well that proves it. I'm so dumb right now I can't even kill this pathetic copy of Superman.
Solemon Grundy: Solemon Grundy not care how stupid he is. Solemon Grundy still crush Superfriends.
Luthor puts his heads in his hands and starts crying as the rest of the Legion make similarly stupid statements.
Yes the Golden through Bronze age produced some good stories but it could produce some really stupid ones. The Hall of Shame in the Wizard Superman Tribute Edition went over some more examples of Superman being written really badly.