Legal Question in Real Estate Law in California

No. You cannot prove your reduced property value was caused by the city's mismanagement of the park. There are many factors that affect a property's value and it would be speculation to say that this alone caused the drop in value. Also, the city is probably immune from this type of lawsuit.

Well, perhaps you could prove to the satisfaction of a jury (or judge) that your reduced property value was to some degree or another caused by the city's "mismanagement" of the park. Also, I don't know of any reason why a city would be categorically immune from all types of mismanagement. I think the suit fails on another ground. You would have to show that the city breached a duty owed to you (i.e., to maintain the park to a certain standard, or however you would phrase it), and I think a court would properly hold that no such duty exists. Without breach of a duty to you, there is no liability, even if there is mismanagement and resulting harm to you.