The Mustang is, was and always will be positioned as Ford's "flagship," if you want to put that label on any vehicle of theirs. Either the Mustang or, to some, the Crown Vic (ducks flying tomatoes).
The Thunderbird (Fox/MN12) was a car that quickly lost its appeal despite the fact that it was a SUPERB vehicle. The big-coupe market was winding down (the Monte, the Thunderbird's main rival, went on hiatus from 1988?-1993), Chrysler never had anything that would stack up, and the entire market was starting to bail out of RWD configurations in most vehicles.
The Thunderbird SC was a vehicle Ford designed with a crosshair on the then BMW 6-Series. It outperformed the 6, out handled it, and out sold it...but that isn't saying much since the 6 of the era was a sales failure (despite its graces). Plus...if you wanted a Bimmer, you weren't going to cross-shop a Ford dealership.
It won Motor Trend Car of the Year in 1989 (specifically the SC, not the Thunderbird...which is odd) and won just about every comparison in any magazine I saw it in. Most notably, in 1995, against a Monte Z34 and Regal GS.
Is it a grandpa car? Yes. Is it a youth-oriented car? Yes. It's quite possibly the most ubiquitous vehicle chassis/platform for a two-door I have ever come across. As much as I need a new car to drive daily, this chassis is just perfect for everything I like and I know it well. I know exactly what it can and cannot do. Maybe I can STILL have my cake and eat it too (maybe pick up a '97-98 Mark VIII?).
So go ahead and tell people that. And if they STILL don't like it...show them how those tail lights look from a stop light. They'll like that. Heh.