She isn't used to any form of real parenting, and is used to things much much worse than time outs. So of course she doesn't want to listen to you and will call you mean.

But answer the questions with her honestly. Explain why there are rules and why it is important to listen to them.

Why do I have to go to timeout?
Because you didn't follow the rules and this is what happens. I need you to follow the rules and the time out is to help you remember them. (Some rules are in place to keep her safe, others so make sure that your property is safe I'm sure... explain which it is to her and why).

Why do I get timeouts?
Because you chose to not follow the rules. (as above, explain what rule she broke and why you have that rule for her).

Why are you so mean?
I'm not being mean, it was your choice to not listen. I'm sorry that you feel I'm being mean, how do you think I feel since you ignored me? (let her answer). Well, I feel very sad and disappointed that you ignored me when that rule is there to keep things safe.

Why can't I do whatever I want?
Because if I'm not here to watch you, you might get hurt. Or you might break something which would make me very sad because they belong to me. Plus, when you're all grown up you're still going to have to do things you don't want to do, like changing poopy diapers (might get a giggle or at least lighten the mood) and you still won't be allowed t do everything you want to.. like you can't rob a bank or you'll go to jail, no matter how much you want the money!.

Just give her honest answers. Rules are in place to keep people and things safe. She doesn't understand that, and that's ok, most 5 year olds don't understand that. They need to have it explained to them, again and again.