It looks pretty cool. There aren't that many blocks, but the pixel art of those present is very charming. But I'm missing several quality-of-life tweaks.

– Select a block or slope by clicking. Cycling through all of them with a space bar is too slow and inconvenient.

– Some sort of mild snapping on blocks or working with curves between the current and last known position. I wanted to remove a big chunk of the world, but if you move the cursor too fast for the framerate to catch up, you'll skip blocks. Same with drawing too quickly.

– Moving the camera is clunky. I'd expect it to work like the Hand tool in graphic programs, this feels like you're using the screen as a gamepad stick with max speed around the edges and zero in the center, no matter if you're moving the cursor or not.

– Just two layers are somewhat limiting. And I can't always tell what the result will look like after I place a block. Especially with the tall building.

– Undo/Redo. At least remember one step so you can take back bad mistakes (a long line of blocks with a single stroke should still count as one step).

– Some sort of export option. Ideally not just a screenshot, but a serialized text version that can be used further (let's say you design a world here that you want to use as a base in a game engine) and re-imported in this world creator to make edits.

– The background clouds don't move fluently at all.

Overall it's a good idea and quite visually appealing, but I don't think it's truly usable at this stage yet.