Do you need reservations if you have a museum pass in Paris/
Some museums require a reservation:
https://www.parismuseumpass.fr/t-en/r%C3%89servation/cr%C3%89neaux-horaires
Yes, for certain museums. If the pass is the Paris Museum Pass, see these pages of the website:
https://static.s123-cdn-static-d.com/uploads/3086630/normal_62569f23e04a1.pdf
"Compulsory reservation of a time slot." means that you have to make an advance, timed-entry reservation.
Also:
https://www.parismuseumpass.fr/t-en/rÉservation/crÉneaux-horaires
I was looking at the Louvre ticketing website yesterday for another question and noticed that there were a lot more time slots available for the regular purchase tickets than for the free museum pass slots. If you have certain days that you "need" to go to some of the museums, I'd book well ahead.
https://www.ticketlouvre.fr/louvre/b2c/index.cfm/home
There are other museums that are not as busy and you'll not need to book.
Are you sure the Museum Pass is a good deal for you?
What museums are you planning?