Yes, definitely. It depends on several factors.
- The distance of the strike, definitely in a close attack the knee would be more powerful than a kick.
- The body part of the receiver, a knee to the groin is definitely much much powerful than a kick to the chest.
- The user, if the attacker is let's say a master in kick boxing which focuses on knees and elbows attacks, then the knee is more powerful than his kicks compared to a Taekwondo master.
At the end of the day, all kinds of attacks has its own advantages and disadvantages and could be more powerful than the other depending on the situation.