If North Korea's largest export is giant statues then yes the economy is in serious trouble. This shows that the country has no natural resources that other countries need or it is not willing to share those resources with other countries which isolates North Korea and hurts the economy overall.
Usually the highest export of a country is food or some sort of consumable good. Something that people can actually use on a day to day basis. I don't know a single person who would want a statue of a dictator. I think North Korea is in horrible condition. Not just the economy but the way they treat their people.
No, this business doesn't show that their economy is in dire straights. Rather, it just shows that they found a way to make money. A country will take advantage of ways to make money just like a business will. If North Korea is making money, they will continue to do this, regardless of whether their economy is in good shape.
Giant dictatorial statues is certainly a niche market, but it doesn't really state much about North Korea's economy as a whole. The country has an inward focused economy, so exports would never be a huge part of it's role in the world, even if it wasn't under a ton of sanctions from the U.S. and other countries.