I like to stick my finger up my ass and smell it

I don't do it but feel free to enjoy your butt as much as you like.
Just please wash your hands afterwards before touching anything in a public place.
It is a health issue.

Your butt has a certain set of microorganisms that are appropriate
to life in that exact enviroment. If you abduct them and put them
in another enviroment like your mouth, it can lead to serious trouble.
Your mouth can protect itself and supports its own micro flora,
but it is not made for entertaining your gut flora.

Stick to smelling it. That's fine.
Please don't put it in your mouth.