EDIT: I forgot to mention, do positive thoughts even do anything? A dwarf can have pages of happy thoughts, but it seems they all go out the window if anything unhappy appears.
They do. The problem is that seeing people killed or seeing bodies generates the most powerful negative thoughts in the game. With the new memory system there are sixteen (8 short term/seasonal, 8 long term/annual) slots for memories. The stronger a good or bad thought, the more likely it is to take up a short term slot, and the more likely it is to eventually take a long term slot.
The negative thoughts from death and corpses are the strongest in the game so they are basically guaranteed to take a slot each. According to the emotions page on the wiki it takes communing with a god, playing with a child, or something equally positive to generate an equally powerful good thought. Good and bad memories return and re-apply their effects periodically.
I've got a fort going with every good and joyous thing a dwarf could ever want and a focus on providing powerful happy thoughts. I'm averaging two to three full strength good thoughts per dwarf. That leaves five or six that would be automatically overwritten by seeing a corpse.
Since each death and corpse-view is a discreet event, killing four goblins (four deaths, then four corpses) will result in every involved dwarf getting whammied with max negative effects over and over for a season, then at half strength after that. While it might be possible to eventually replace the bad long term memories with extremely powerful positive ones, that would be the project of years and would only be possible by turtling and removing all sources of danger. Even then I think the replacement of long term memories is chance based, so who knows how long it would take.
So the very short version is yes, good thoughts do have an impact, but right now they are completely overwhelmed by the bad ones.