We have several definitions running in this thread. While it seems we agree that a lot can be done to beef up the fruit, there are limitations. My comments are with respect to grape wine, as non-grape is too wide a subject.
Sugar, acid, tannin, body etc. can be adjusted, but those are modifiers. The fruit must have a basic level of constituents with which to work, and those cannot be improved without adding something like concentrate. Blending works to address (mask or cancel) flaws, but it doesn't improve the underlying basis of the wine, e.g., the qualities that make Cabernet Sauvignon smell and taste like Cabernet Sauvignon.
IME poor quality fruit will not make better than an OK wine. On several occasions I purchased low quality fruit and could not make a good wine from it. Ya gotta have something to work with, and I didn't have it. In hindsight the one thing I might have done to improve it would be to use grape concentrate to chaptalize rather than sugar.
Decent quality fruit makes a good wine (good varietal/blend characteristics), and can be made into a very good wine with care and effort. The better the fruit, the higher the potential.
Consider that the best wineries cannot
consistently produce classic wines. They have the best fruit, highly experienced winemakers, and the best facilities, and the most they can consistently do is make outstanding wines. While the folks on this forum would be quite pleased with regularly producing outstanding wines, for professionals it's a disappointment, especially on the balance sheet. For us this illustrates that we need to properly set our own expectations.
FYI, I ballpark judge wines on the Wine Spectator scale, which I listed in a
post I wrote a few years back.