Yes you do have to remove the motor mount place a jack stand underneath the motor and take out the two bolts on the passenger side motor mount then the belt will go on
How Do I Replace Serpentine Belt For A 1999 Buick Park Ave?
I have a similar Buick to yours with a v6 engine and I didn't have to remove anything to replace my belt. Unless the belt is on the engine and the mount is on there in a way that it is inside where the belt runs, I would say no. I am assuming you have a v6 engine in your Park Ave.
Find an open end wrench 7/8 or so what ever fits alternator bolt on the front , turn left and that will make your belt loose . Make sure you look at the diagram on the hood which way it goes back on , use your phone to take a pic how it goes on. Do the same thing when you put back on .