"I'm starting to look for a new job. My main reason is that I'm burning out in my current position. I work 8-5 and am on-call nights and weekends for after hours work. I can work from home at those times but I can't make plans or go anywhere because it's so involved.
When I get asked why I'm looking for a new position is it okay to say that I'm looking for better work-life balance? Is there a better or more appealing way to state that?"
When interviewing for any job, focus your motivation on the company you are interview with, NOT the fact that you need to find new employment. Think about a date with someone: you wouldn't tell someone on a first date that you are seeing them because your ex-girlfriend is intolerable, or that you're seeing them because you're desperate. You would tell your date you are out with them because THEY are amazing.
Try to get a clear idea of work-life balance early on - and avoid those positions. Ask about work requirements carefully - you don't want to seem like you are trying to work as little as possible. Explore what types of work utilize your skills that don't require An-call work. It might take a slight career shift, but might be necessary to avoid that amount of time commitment.