I'd probably just move on personally, since your interests and her interests aren't likely going to align. But if you want to keep going with it, you can always try a response that changes the frame.
On the topic of "seriousness":
"It's only wasting time if you're not enjoying yourself. [followed by a question that changes the subject]"Just deflecting the topic to something else:
"I've been called a lot of things, but never a waste of time. I challenge you to grab a drink with me and still hold that opinion by the end of it!"
"Life's too short to take it seriously, so why take dating too seriously?"
"What? You went to my profile? Most girls don't get past my profile pic before clamoring over themselves to message me back :P"Just make sure you control the frame and she'll respond back to you. But like I said, when your interests don't align with your targets your just setting everyone up for a fall. If a girl is serious about her position of not wanting a fling, I respect that and make a decision as to whether I'm interested in something a bit more serious or not.