I'm thinking of placing a stuffed toy in my sister's hamster cage
Why would you put it in the cage?
I am just bored I guess. I know this will be funny.
It is not okay to put things in your sister's hamster cage, it could damage it and hurt her feelings. You would have to get her a new one.
I'm going to do it anyway and I don't care what people think.
If the toy is oversized it might damage the cage. Have you thought about what might happen to the hamster? It might get scared or possibly choke on stuffing.