Grégoire Leprince-Ringuet has a love scene with Louis Garrel 74 minutes in the movie. It is a very sensual scene but without full nudity. However when they wake up in the morning in bed, we get a good view at Grégoire Leprince-Ringuet's ass and hairy crack.

Ma Mere is a disturbing movie with a lot of nudity from Louis Garrel.
14 minutes in the movie, there is a bunch of young guys mooning at a dinner. One guys shows his uncut dick and big balls too.
21 minutes in the movie, Louis Garrel gets naked and we see his butt.
24 minutes in the movie, Louis Garrel is jerking off. We can't see his penis, but we see his butt when he finishes.