60% is the cap for new towns (so that they're not too easy to obesify). But the country's rate should continue to increase beyond 60%. At 20 towns, you should be pretty close to obesifying the country. Check the news section for the country's current obesity rate.
When you reach 99%, you unlock a final set of newspaper stories & that's the current end of the road. But I plan to add much more to this in a future update (involving an organisation trying to fight back against Dizzy's that you can help or hinder).