Bitter Choco Decoration depicts a shy person who eats away at their self in order to fit in with their company, and in the end, they're just a hollow shell that's just bitter and hateful.

The servant doesn't show up to serve food at 2:59 because the master had already eaten up their individuality. There is nothing left to serve to their master.

At 3:08 the master looks lost, with a vacant look in their eyes when greeted by one of their friends. A malevolent distortion of the master's self flickers at 3:15 presumably because of their bitter regret in changing.

The master's friend unintentionally reopens old wounds when they say, "Now I remember! You’re the one who always seemed super serious- Never mind, forget I said anything".

That's just how I interpret it though.

I liked it mainly for the animation, it's what makes the song interesting.

This lyric page tells it better.