So I am gonna try out some Super Mario World categories, but before i start, is it allowed? Asking bc I haven't seen a bunch of people using it.

It depends of the game, some games allow it, some not. You'll find it either in the rules or on the Forum, for SMW it's here: https://www.speedrun.com/smw/thread/8w7yc . If it's nowhere listed, just ask on the Forum or on the Discord if there's one.

Here is the thread of valid emulators for SMW https://www.speedrun.com/smw/thread/8w7yc (BlueYoshi’s link has a rouge period at the end)

You’ll see that farther down the page someone asks about OpenEMU. It uses the bsnes core so you’ll be allowed to use it for runs.