Not too bad...
The cheesy dialogue works for me... there's nothing like the epitome of evil making wise cracks and being generally dumb. :)
The one thing that is a little annoying to me is that this is supposed to be a tower defense game, but the gameplay makes it a trial and error puzzle game that is unwinnable.
For example, if for some reason you have a shielded enemy that gets through, and you have have your mutated minion that's already been used, you cannot move that minion back to attack the shielded enemy. But how can you move the minion around to each room before he/she becomes your minion? That doesn't make sense. I have three suggestions:
1. Allow Cthulhu to drag minions from one room to another (once they are mutated they are your minion). It makes for easier gameplay obviously in the beginning, but might set up a frantic finale at the end.
2. (more difficult) Instead of letting Cthulhu drag the minion wherever, only allow him to mutate an enemy at the point of mutation. This will require a LOT more strategy, and may be more in the spirit of what you are trying to do. Perhaps the challenge becomes on YOU to create a challenging wave that seems impossible, but with the correct timing and placement of minions, it can be done.
(Note: you can do either of these, and create an easy/medium/hard level... I'm a big fan of different challenges within the same game... it also allows you to add more silly dialogue if you want)
3. Keep the format you have exactly the same, but create fast forward scene when it's obvious that the player cannot win. Since you are going for a silly theme... insult the player with something like "Oh my god, what have you done, Cthulhu"? That may seem complex, for your program to identify that, but its been done before and makes the point of emphasis for the player to make sure they plan ahead next time for the last enemy.