Other sports cars from 1964:
snip
I could go on and on and on. I'm not even trying to argue that the Mustang was a sports car, but it was, and was intended to be a 'sporty' car from it's inception.
From Merriam-Webster, Sporty:
165bhp in a car weighing 2,450lbs is about what a current Miata is. If you are judging on power to weight alone. The Miata is a sports car, right?
My RX-7 was designed from the beginning to be a sports car, and it's power to weight ratio, as stock, wasn't that good. My car also has a solid rear axle, and suspension components mostly truck derived. It's still a sports car.
I am not using power to weight as the metric for sportiness. Power to weight
can be sportiness all on its own such as the case of a big block Charger or GTO, but the Miata and RX-7 get their sportiness elsewhere in the handling department. The Mustang was lacking in both departments.
It would be worth noting that all of those examples you posted run sporty suspension and are far lighter than the Mustang was.
At this point it comes down to what your interpretation of that definition is.
My interpretation of "sporty" is having enough features and design points on the car to attract drivers and to provide pleasurable, not necessarily speedy, driving dynamics.
Having driven a stock 1st body style Mustang of similar setup as a 1964-1/2 (both with an I6 and V8) on windy roads, around town, and on a track and having worked on the components of that car that contribute towards the car's dynamics, I feel qualified to say that the original Mustang is far from offering a sporty experience.
If your interpretation of "sporty" is grounded in looks, then the 1964-1/2 Mustang is sporty.
According to the MW, the original Mustang was sporty because of that handy "or". However, I'd argue that there are dozens of cars that resemble a sports car in style that you would hesitate to call sporty cars. A Kia Sportage
resembles a sports car. Does that make it sporty? According to the dictionary it does.
It was 1964, automotive technology wasn't exactly stellar back then. Plus the Mustang was designed with cheapness in mind. They wanted the average person to be able to afford it so they weren't going to go crazy with stuff not from the parts bin.
Stuff that makes a sporty car today wasn't what made a sporty car 50 years ago.
I know. I point back to the fact that even for 60's the Mustang's driving experience and technology was particularly unsporty.
It was a sporty car though compared to other vehicles on the road, it just wasn't a performance car. Would you just prefer if I said it was designed to be a fun car?
Fun would work. This goes back to the definition section in my post above. According to MW, a car is sporty if it resembles a sports car in style or performance. The Mustang certainly
resembles a sports car in style, possibly in performance too depending on your standards. In my experience, the early, early Mustangs were basically family sedans that had undergone some plastic surgery to appeal to a younger audience. It wasn't until later that they developed the personality we commonly think of.