So umm... it looks like when Don't Dig Up is activated with the tool to generate a world it doesn't actually activate remixWorld in the code. There are a ton of changes that are tied to actually having Remix seed activated that makes sure that the seed works properly.
Here's a link for what Remix seed does:
Don't dig up
When I spawned in in my new world made by the tool a Demon instantly spawned and blasted me to death. The only enemy that is supposed to spawn at the main island in the Underworld on Remix worlds are Lava Slimes that have the exact same stats as a Blue Slime + don't drop lava on death. These Lava Slimes are also supposed to drop Gel.
So you'll have to make it so remixWorld or whatever it's called in the code is activated for these worlds generated by the tool, because otherwise it's unplayable. Can't have regular Underworld enemies constantly spawning at the player spawn point and one-shotting the player instantly. Might also wanna get rid of the lava underneath the Ash Blocks at the main island in the Underworld on Remix seed since when For the Worthy is activated Ash Blocks can randomly fall when mined + lava deals 200 contact damage on Remix seed unless Ash Wood armor is worn.
I haven't gotten around to testing much else currently, but I'll keep you updated if I do.
But basically, you need to make sure the tool activates remixWorld (whatever it's called, though I'm fairly certain it's called that).
EDIT: Maybe I generated the world wrong somehow? Because I see in your screenshot that you have a black sky like in Remix, I did have For the Worthy activated at the same time if that makes any difference, just so you know.
EDIT2: I disabled Celebration, FTW and both ores and evils. Now I managed to generate an actual Remix world. Looks like there's something that conflicts when using a certain setting together with Remix that breaks it.