In terms of what you can do to make your DD more resilient . . . it's a tough one. I think what I'd suggest is being sure to keep the lines of communication open with your daughter. Ask to hear about her experiences and just listen.
A big part of it will be that she feels invalidated, and other children aren't speaking up for her, even if they think what's happening is wrong. She needs an outlet to just talk about what happened and process it, first of all. Don't rush to get angry. Let her explain what happened, and ask her how she feels about it. Listen, listen, listen.
My second piece of advice would be to remember you're dealing with a child. They don't process the way adults do. When I was bullied as a child, my mother was quick to dismiss it by telling me the bullies were "just jealous". She would jump in with this statement very quickly in the telling of any story, and would seem quite impatient. It might have been anger at them, or frustration with the situation, but as a child, I didn't see that. I felt she was impatient with me. That I was being silly. That I shouldn't care.
My mother had never actually taught me resilience, but seemed to expect me to have it. I 'knew' I 'should' be able to brush off the bullying, but no-one had ever shown me how to do that. As far as I could tell, I was just supposed to not have feelings about it. If I cared, I was being oversensitive and weak. The result was that I felt worse, clammed up, and stopped going to her for comfort.
I'm not saying don't be angry at the situation. But it's better to keep that anger in check in front of your child. Listen to them, validate their positive traits, and make sure they know there's no truth to the bullying and that it's not okay. But resist the urge to let your emotions out in front of them. I think some mums believe ranting and raving about what a little bitch the bully is will make their daughter feel better, but often it just keys everything up even more. They think their daughter wants a tiger mum to defend her. But what she actually wants is a steady, comforting presence, who seems rational when she says the bully is wrong. Someone who isn't just saying it out of maternal bias.
The parent who says "No-one talks about my daughter like that!" and hares off on one, is making it all about their feelings, and that can be quite unsettling to a child. Versus the parent who says: "That isn't true, and it's not right someone spoke to you like that. Here's what we're going to try first, to fix this. And if that doesn't work, we'll try something else."
Try to empower your child to resolve the conflict herself, and try not to demonise the other girl. The more your daughter can understand her bully as just a person with flaws, who may even be capable of change, the more in control she will feel. Show her the power of pushing back calmly on bad behaviour. "Your top is babyish!" "I like it." "It's babyish!" "It's my top, and I like it. I'm bored. Do you want to do some painting? I like your paintings." Etc. Teach her to not give the nasty comments any oxygen, and learn to redirect the other little girl with positivity instead. Sometimes, that's enough. And if it isn't, she'll feel stronger because she tried, instead of letting the grown-ups manage the situation for her.