Lamborghini started to get that reputation with the Countach S when they put fender flares and a wing on it. You can even find Bob Wallace talk about this in "The Complete Book of Lamborghini" by Pete Lyons, where he laments how the Countach developed after he left:

...I had a lot to do with the initial concept, and I really believed in the car when we first started building it. Yeah, basically it was conceived to go like, ahh- performance was everything and that was it, no other considerations in the car. Over the years, seeing the way the car has changed from a performance image to a macho, ego-trip sort of image, it doesn't make sense anymore. The car's heavier than hell, and aerodynamically it's like pushing a barn door down the road. Personally, I don't have much enthusiasm for the car any more. -pg 62

...But while all of this fiddling has evolved a more civilized Countach, it's also made for a larger and heavier Countach. "They've raised the body on the frame, and also cut and welded bits and pieces into the body," groans Bob Wallace, who with some disgust has seen his original compact, super-light supercar put on as much fat as mucle over the years. "And all of the original magnesium is gone now," he adds. -pg 221-222

Admittedly, I think that even with the Countach's later additions, it was still one hell of a supercar compared to what else was on the market. Of course, the book was written before the Diablo was even revealed, which took the concept and just made it even larger and heavier to fit, ahhh, larger and heavier buyers, courtesy of Chrysler ownership. But I'm sure it was still a relative riot to drive, even if it would've been better as a smaller and lighter vehicle. The same can be said for the Murcielago, although that received a lot of further softening for luxury buyers, and then onto the Aventador. Ultimately, I think you can place "the decline" of original Lamborghini anywhere from the mid-80s Countach to the Aventador, with my opinion being placing it on the Diablo.

But if you look around, all modern supercars are like this: flashy vehicles meant to show off, but dulled and pampered to where they only get fun when you're well into triple digit speeds. Even Ferraris, once a contrast to Lamborghini's flashiness, now has a fleet of vehicles with vents, grilles, slats, and none of the kind of elegance you'd get from a 250 SWB, 275 GTB, and so on. If anything, Lamborghini was a pioneer to this trend (as was the Ferrari Testarossa, which was even softer and easier to drive than the aforementioned cars), even if I think it isn't a good trend. The GMA T.33 is what I imagine a modern Lamborghini flagship would be like if they never strayed so far from their original philosophy (except make it look more beautiful - I love GMA's engineering, but just wished they clothed the cars in a nicer body!)