Kody slicked his hair into a ponytail for Janelle’s anniversary dinner because she likes it that way. He tweeted that she’d go “goo goo eyed” over it and based on her own tweets, she did. He also said he loved the way it looked.
When Meri straightened it for her birthday date, he hated it. Said it made his hair look too thin even though she styled it to match her “Keith Urban” fantasy.
Then in Season 8, he was in Meri’s house wearing the ponytail again… and she snapped: “You don’t get to wear that in my house.” She didn't even say it looked bad. In my opinion, she didn't like it because it reminded her who he actually wanted to impress.
The levels of favoritism have always been loud, even when it was just about hair.