The rules term "fighter" includes both aerospace and conventional fighters per page 20 of Total Warfare.
As an advanced, optional rule, players are expected to use a certain amount of discretion and common sense in its application. Obviously, the Unstreamlined quirk is unsuitable for use on conventional fighters or fixed-wing support vehicles, but it is not technically impossible to design an aircraft with such poor streamlining it's unable to take off. Considering the near complete lack of any utility whatsoever, it is advised that players attempting to design something like that be referred to the Combat Vehicle design rules or the pages of XTRO: Boondoggles.
