"Tang" is their food preference. The normal brown food corresponds to Normal tang. Cake or corpses (I guess depending upon the mindset) corresponds to Gourmet tang. Garbage to Poor tang. IME it takes three meals to change from one tang to another. But yukkuri will only eat garbage with 100% hunger and (I think) 50%+ damage.
Marisas are the worst for complacency due to the strong will of Touhou canon and yukkuri fanon. Marisas lose more complacency than others relative to the problem and get pissed-off quicker. I always have morale problems with baby Marisas because their parents (particularly if a Marisa) very rarely clean them after shitting. And Yuunbas never get their fast enough. If I've got one I want to keep, I usually give them an Accel Amp and sell the rest.
I've also notice that a yukkuri in distress affects the disposition of all. Even if it's a shithead getting punishment for trash talking, once they get above a certain amount of damage, none will respond if you tell them "take it easy". I haven't noticed it'll drop the morale of the rest, but I never put up with shitheads long enough to find out.
Tang is quality of filling. No idea why, but It was technically english already and fit so I didnt translate it. Basically, better Tang will result in better selling price and better quality food from mulching (I think). Try abusing while keeping alive to raise tang (I think)