If we consider the handheld market, which only consist of 2 devices, the 3DS (45 millions sold) and the VIta (4 Million sold), it is pretty clear that "specs and hardware power" mean jackshit to the customers and it's no argument for or against the success of a device, yet it is often times used by people to mock the 3DS.
It is even more apparent if we take a look at every handheld since the original Gameboy (which didn't have great specs either). Hardware power has NEVER meant anything to most of the customers of handhelds, only software and innovation have.
Calling something under- or overpowered makes only sense if we consider the purpose something is designed for. Let's say you have a race car and try to compete in the formula 1. The goal is to become the NR. 1 in that sport. If you have a car that is 100 PS slower than everyone else, you'll not gonna win. It's no doubt your car is "underpowered" - it has not enough horepower to successfully compete, let alone win.
The handheld market is similar. The goal is to be the market leader and make the most profit. Specs don't matter. Nintendo, with often lesser spes than the competition has always won in this market, simply through better software, stronger ip's and innovation. In that regard calling their handhelds underpowered doesn't make sense.
The 3DS is not underpowered, it is powered just right, while the PS Vita is clearly owerpowered for its purpose. It has better specs, but those didn't help it achieve the goal it was build for.