I added a negative value like this:
<Building_YieldChanges>
<Row>
<BuildingType>BUILDING_CUPWONDER</BuildingType>
<YieldType>YIELD_HAPPINESS</YieldType>
<Yield>-15</Yield>
</Row>
...
...
</Building_YieldChanges>
When I try it though, I don't get any unhappiness. Did I misunderstand your instructions, or is there something else wrong here? When you say "displayed a little funny", is this not what you meant, correct? Since this is not a display error from what I can see. I cannot see any unhappiness added while looking at the happiness tally while in-game.