Just wanted to clarify if using a keyboard with my switch is allowed.

Is this possible?

Its possibly. Not the best for switch, but holding the pro controller in a dpad position hurts my hands a lot, so I'd prefer to use a keyboard

It would be better off playing on an emulator if you're already using keyboard.

I don't know why it wouldn't be allowed. There are no restrictions on controllers - other than ways that give you a categorical advantage like turbo functionality.