QR codes should be more than a rehash of your website. This is especially true if your site isn’t even optimized for mobile viewing. If I’m walking by your billboard and catch the QR code I want to see something that catches my eye and makes me stop walking for a second. Taking me to your same standard website may make me stop walking, but only long enough to close my browser and put my phone away.
Think of it this way. You’re using QR codes in the first place to get folks who are engaged in another activity, in the case of a billboard walking to their destination, to explore your brand further.
Even in the best of situations scanning your code isn’t the primary focus of the user at that moment. Reward them for doing so. A mobile site dedicated to the add they’re looking at or a YouTube video following up is a great way to do so. For example, if you’re a college trying to sell the great job placement your ad offers link to a video of alumni doing that job and talking about their experiences at your school. Just don’t take them to the same content they can get on their computer!