I agree that there were NEVER any FACTORY convertibles. There are, however, a number (i.e. not one offs) of Miradas that have been shown here that are THIRD PARTY conversions.
The reason I thought it was a convertible has very little to do with the top itself, but mostly to do with the trim around the vinyl roof. It is not the same as my 1981 Mirada CMX, which has a similar roof. By that, I am talking about the trim that surrounds the vinyl top, as in the edging and overall shape. On my car, at the top of the windshield, there is a small section that is body coloured. On your car, there is not. It may be a year to year difference, I don't know, as I only have the one 1981 Mirada as a point of reference.
As for the body style, this is a great body style, and while the collector market may not deem it to be "collectible"/"valuable"/"desirable", I certainly do (or I would not have bought one). It is one of the better looking cars of the early 1980s, unless you were into "disco" cars.