The worst cases during that period were Chris Hodgetts' back-to-back titles in 1986 and 1987 because he did it in the lowest and slowest of the four classes.
If you were to award the BTCC by the class of the fastest cars with the biggest engines during the 1980s, i.e. the ones we would think of as "really" winning the races, it would look like this:
1980 Champion - Gordon Spice (Ford Capri) Actual Champion - Win Percy (Mazda RX-7)
1981 Champion - Pete Lovett (Ford Capri) Actual Champion - Win Percy (Mazda RX-7)
1982 Champion - Jeff Allam (Rover Vitesse) Actual Champion - Win Percy (Toyota Corolla)
1983 Champion - Andy Rouse (Alfa Romeo GTV6)
1984 Champion - Andy Rouse (Rover Vitesse)
1985 Champion - Andy Rouse (Ford Sierra XR4Ti)
1986 Champion - Andy Rouse (Ford Sierra XR4Ti) Actual Champion - Chris Hodgetts (Toyota Corolla)
1987 Champion - Tim Harvey (Rover Vitesse) Actual Champion - Chris Hodgetts (Toyota Corolla)
1988 Champion - Andy Rouse (Ford Sierra RS500) Actual Champion - Frank Sytner (BMW M3)
1989 Champion - Andy Rouse (Ford Sierra RS500) Actual Champion - John Cleland (Vauxhall Astra)
So you would have had Gordon Spice and Pete Lovett winning their only titles, Tim Harvey with an extra title, John Cleland with one less, none for Chris Hodgetts and Frank Sytner, and Andy Rouse with three more bringing his total to seven.
The Jim Clark Trophy, but yeah. He scored 95 points whereas Piquet "only" scored 73.