I remember somewhere in one of my high school US history classes that I read that, during the Civil War, it was a common practice for wealthier citizens to picnic on a hill and watch a battle. Was this true? Was this common in other eras as well?

This question has been mostly answered before: https://www.reddit.com/r/AskHistorians/comments/2h0jaf/did_americans_during_the_civil_war_really_try_to/?st=JKA19VNV&sh=bff09d22

This was something that mostly just occurred in the First Battle is Bull Run (Manassas). Washington, DC residents believed the battle would just be a rout for Union forces, so they gathered on the hillside to watch. The battle turned out to be the opposite.

Part of the motivation for certain civilians to spectate the battle was for the political networking opportunities, not just to watch a battle for the sake of it. It was also an opportunity for journalists to document what was going on during the battle.

During the civilian retreat, New York congressman Alfred Ely was captured by confederate forces and spent several months in a military prison.

There’s little evidence of this happening during other parts of the war.

Source: https://www.smithsonianmag.com/smart-news/was-first-battle-bull-run-really-picnic-battle-180964084/

https://www.senate.gov/artandhistory/history/minute/Witness_Bull_Run.htm