Superman is smart..ish. It's more about the speeds of his brain as opposed to just being naturally smart. If you could read every book in the world in a matter of a day or so and retain all that knowledge, would you not be insanely smart?
As for inventing, he has used kryptonian tech to create things. But it's not like he simply just pressed a "create this item" button. He built the stuff using tech, in the same way Reed Richards builds shit with the tech available to him. Of course Supes isn't on the same level, but he has created the Superman robots. He also created a machine that turned the entire Phantom Zone into a paradise dimension.
All that is for post crisis btw. Pre Crisis had time machines and time windows and all kinds of wacky stuff. Even Superboy had a time machine and basically showed a young Bruce Wayne an image of them in the future as Batman and Superman. Yes Superboy could of probably used this device to prevent the deaths of Bruces parents but..well..we do not speak of such negative thoughts.
He's quite smart. Just not "Batman smart". I don't think I'd rank him in the top 10 smartest but he did create that device in "For Tomorrow" that transformed the phantom zone into a beautiful place and transported all those people to it during "the vanishing". He's also very battle savvy when he has to be (which is rare, of course, since his raw power is usually more than enough to beat most opponents) and has used his powers numerous times in creative ways to accomplish things whether that is beating an opponent or saving people.
Now, if we're talking PC Supes then he'd definitely be in the top 10 smartest DC characters.
If written correctly he is def up there, over guys like Batman, Terrific etc. But these 10 are def smarter from DC not including night/ or omniscient beings
Brainiac 5
Brainiac 1
Brainiac 2
Brainiac 3
TO Morrow
Metron
Luthor
Caulder
Magnus
Ultra-Humanite (honestly as the original Superman villain he has feats even better than Lex in science, and has outsmarted Supes multiple times)