I see what you're saying Igo kyu. If a designer is given a certain powerplant to work with and they produce a design that is overweight for the powerplant you could say that their
design is overloaded, and that may be their own silly fault.
However, in common aviation terms, if you are calling an
aircraft underpowered or overloaded I believe those terms mean two different things. An overloaded
aircraft is usually one which is operating
above its design weight, or at least above its design weight for a given situation. In comparison, an underpowered
aircraft is usually one which lacks performance
at or below its design weight.
And for the latter case, the cause may well be, as you suggest, the fault of the designer. It may also be the fault of whoever insisted on adding extra weight/drag to their design prior to production. Or in some cases the engines that the airframes were designed for just didn't come up to expectations, but were used anyway until something better was available. And finally, use of the term underpowered can be a relative thing. What was considered reasonably powered at the beginning of a design process, may be considered underpowered compared to other aircraft by the time it reaches production.
Of course if you are flying an aircraft which is both underpowered
and overloaded then just taking off and landing safely may be a challenge!
