I want to write a sad scene where a mother finds out that her son passed away. What things/clichés should I avoid in order to not make it overly dramatic?
By "over drama" you probably mean "melodramatic." The following article gives us several clues to keep it dramatic rather than over-the-top melodramatic: https://screencraft.org/blog/the-single-difference-between-cinematic-drama-and-melodrama/
Drama depicts realistic scenes and character actions while melodrama does not. So make sure your scene properly grounds the mother in reality.
If your scene can be summarized as "Mom gets bad news, and feels sad." I would skip that scene.
There's no chance for any character to meaningfully change the way this scene plays out - the news is bad, and the that's that. There's no stakes. There's nothing to risk, and nothing to gain.
Since there isn't any source of tension in the scene, it will inevitably feel weak.
Contrast the "Mom gets bad news, and feels sad" scene with on like "I need to tell Mom her kid is dead, but lie to her about how he died."
The second scene has stakes - the POV character has to lie, and lie well, otherwise they will be caught. Mom's pain becomes meaningful to the reader, because the POV character has to look into the grieving mother's eyes and lie through their teeth. It affects the POV character. (or it doesn't, which could tell you a lot about the kind of person they are.)
The Indirect Approach
If you don't have good stakes for a conversation with Mom, you can show her pain indirectly. Pick a different scene, and insert her grief into it. You could:
- Start the scene directly after Mom has been told. Everyone is ready to move on to "What do we do next?" Describe Mom wiping her eyes. POV character thinks She took that well. Someone escorts her out of the room to grieve somewhere else. Mom has a lot of the info they need to make decisions, so everyone is keenly aware of her absence.
- OR Someone volunteers to break the news. POV character tries to convince another character [something]. At just the right moment in the conversation, an anguished wail pierces the night. Everyone knows that Mom just got the news.
In any case, something else is providing tension in the scene, and the reader just gets a glimpse of Mom's pain rippling out into the world. The reader will care more about a little peak into grief that occurs during a strong scene than a long view of grief in a weak scene.