O.K.,you know that in the movie,whatever he creates disappears out of thin air once the mask is removed?But remember those mechanic guys?The Mask shoved mufflers in their asses.By the next day,when the mask is not active,the mufflers are still there.Yeah,they didn't disappear.
I guess he grabbed them from the their own garage maybe?
He showed up to their garage with the mufflers already in his hands.
There is a deleted scene where the Mask grabs some mufflers from a pile, so they were supposed to be real.
Even if he grabbed them from a pile,I'd doubt that the Mechanics had them all shiny and cartoon looking.(This is just what I think.)
They actually filmed the sequence but it was cut, I think because it made the Mask too dark and it interfered with the pacing of the scene (I've tried editing it back in and that's what I felt from it.). Though they are shiny (They could've recently come off a new, junked car.) they don't look cartoon looking while the Mask is holding them, but they do become more CGI as the whirling CGI Mask tornado wields them a few seconds later just before attacking the mechanics. So the cartoony look of the things in that sequence is just down to the limited CGI graphics technology of the time.
I guess he came prepared xD
Maybe he didn't create them. Maybe he grabbed them before showing up at their garage.