That is certainly one of the major disadvantages of feeding live prey (aside from the fact that the mouse could do a lot of damage to the snake). But to answer your question, feeding junk food to the mouse is not a great idea. Snakes get some of their much-needed nutrition from the contents of their prey's stomach, so you would in effect also be feeding your snake junk food. And - junk food will put on fat rather than protein to the mouse, which again will not be as healthy a meal for your snake.
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