Z-Moves that boost the user's stats, such as Z-Rain Dance (which raises speed by 1 stage) and Z-Happy Hour (which raises all stats bar acc/eva by 1 stage), ignore abilities that influence stat changes (such as Contrary and Simple) in official games. However, in Pixelmon, these stat changes are influenced - using Z-Happy Hour as an example, Contrary causes it to lower every standard stat by 1 stage, and Simple causes it to boost every standard stat by 2 stages (in both cases, it should only be boosted by 1 stage).
It is worth noting that there are certain exceptions to this rule - for example, Extreme Evoboost is influenced by both abilities.

Thanks for the report. Good catch! I'll put this on our internal tracker to be fixed.