All air-breathing engines derate with altitude, be they jet, turboprop, or piston.
That the propulsion is adequate for high-speed cruise at efficient altitudes does not suggest the aircraft is overpowered. Cruise is a primary design point for most aircraft (Though there are exceptions that focus more on loiter). Single-engine performance often sets design criteria, however, which does indeed tend to result in "overpowered" aircraft. But it beats the early light twins, in which it was said that the second engine was there to get you to the site of the crash.
According to my trusty Raymer text, a typical high-bypass turbofan (Jet) engine will derate to 20-25% of its rated static thrust in 0.8M cruise flight between 30,000 and 35,000 feet. A turboprop engine such as that in the King Air tends to see equivalent shaft horsepower between 60-80% of the takeoff value in cruise.
Naturally aspirated piston engines taper off from rated power at sea level to in the ballpark of 50% of rated power at about 15,000 feet. If you turbocharge or supercharge the engine, it is able to maintain rated power to some altitude (Raymer says for the TO
-360, that altitude is 10,000 feet or so), whereafter available power tapers off at the same rate as the naturally aspirated version of the engine. There is also such a thing as a "turbonormalized" engine, which does not increase the rated power compared to the naturally aspirated version, and the turbo is used only for the purpose of maintaining that rated power to a higher altitude.