Do you mean that she says stuff like that during a conversation with you, or does she text you that she's bored before there's any conversation? (I don't know how to say it otherwise, but I hope you understand what I mean)
In the first case, you should probably do what FCgame said. Make the conversation interesting. Ask things about her life and pick in on them. Also, making statements help keeping a conversation going. Stuff like 'what are you doing' isn't probably going to work, since she's obviously doing something which made her feel bored. Talking about a funny movie you recently saw, about a situation in which you both laughed your asses off, a happy/funny memory... If you already know the girl well you can joke about something she said or did, I don't know, make her feel less bored, show her you're interesting. If she has nothing to do, you can say stuff like "I was just going to the park, wanna join me?"
In the second case, out of all peoples, she texts YOU when she's bored, which means she's thinking of you. You can ask her what she was doing and if she's planned anything for the rest of the day. You can always ask her out to go hang out togetter. She texted you, so she must like you (or not hate you at worst).
You haven't given much info about your connection to the girl, so you have to determine whether or not to meet up with her. Hope this helped
