Here's something I noticed by monitoring Lisa's mood with the cheat menu:
For those wondering WHY Lisa's mood is hard to keep good is because when she asks you to turn around or wait outside, if you do anything other than
"Turn around and Not Peek" only at 08:00 and 23:00 you loose mood points even if it says her "Spirit is Up", otherwise at 15:00 you will either loose 20 mood points or her mood points goes to 20 (Not Bad).
Edit: Not sure why just yet but I just checked my Day 22 and my Day 25 saves and in Day 22 you still gain mood points during the 15:00 for Turn around and Not Peek but starting in Day 25 you start to lose points no matter what at 15:00.
Also noticed in the game code the Mood Neutralizer.
Every time you do something that advances the time, each girl’s mood moves closer to 0 (Neutral), meaning if her mood is greater than 0 (Good, Very Good, Wonderful) she will lose mood points but if her mood is below 0 (So So, Bad, Very Bad, Terrible) than her mood points will increase.