I want to submit a subcategory run of super mario world, in which basically its a level 1 run. How can I request to submit this?

Talk to the moderators of the game

https://www.speedrun.com/smw/thread/jbwqk

Level 1 isn't that hard to beat. How would this be a good idea?

I'd say that would rather fall into IL (individual levels) boards than a specific category.