Somebody wanted a character design for free but I am thinking of giving them a rubbish one.
Really. Why do you think you need to retaliate like that?
I don't like them anymore so I decided to do this.
I've been salty towards people that I dislike, but I don't think designing something badly on purpose is a healthy way of dealing with people even if they were trying to take advantage of you by wanting it for free. What I would do if I were you is to demand payment about the character design, instead of actively just not doing the best character design that you can.
I just wanted to cause a problem for them.
But you are also causing a problem for yourself. You would still be wasting time drawing something. It would just be a better use of your time to not sabotage the drawing because you don't like them.