Even superplants have competition
In the real world there are plenty of examples where certain invasive plants can be considered 'superplants'. It is important to note that there's no hectares of superplants that have out competed everything in an area. Sure you can see some areas where it's a real problem, but a full blanket of mint from horizon to horizon, and the horizon beyond, and beyond that are unlikely.
The reasons are simple. Survival of the fittest. The existing plants have two advantages. The first is that they are already there, limiting soil and nutrients. They might shade an area, depriving it of vital sunlight or even the fungi in the ground might not cooperate with the mint. The mint needs to adapt to a wide range of situations, soil compositions, water requirements, sunlight, water depth, droughts. It isn't unconceivable that it can adapt to a wide range of situations, but during the adaption time it is vulnerable. Not to mention that any plant could do the same, but hasn't.
Finally such a plant would get predators at a certain time. It is abundant and has energy. Something will eat it eventually.
How to still get a 'superplant' that covers the world.
Your best bet is to not directly compete, but cheat. First of all, you make a plant that easily adapts. This increases complexity, and thus makes it harder to truly compete. But you have two extra weapons. The first is that there's many kinds of the mint created, able to cross breed easily, allowing the flexibility to reach further heights and many kinds are already prepped for many environments. This will make competition much easier.
The second is that it attacks anything that isn't one of those mint plants chemically and virally. Your plant is able to sample the plant life around it and create chemicals as well as bacteria or viruses to the detriment of other plants. Think of simply increasing the acidity of the ground by leaps and bounds. A virus might not kill another plant outright, but as it's spending so much energy to defences it is easier to compete with.
It will have a bigger chance to disrupt ecosystems, causing them to collapse. I still think it'll not cover the world, but it has a good chance to be as prominent as grass while edging out other plant life. It can still be the 'hellscape' of mint you want to be, though less prominent.