No, I actually think having stuff like tuner cars (some of them I wouldn't really specify as tuners, such as the Spoon Sports race cars for example) is actually part of automotive culture. This especially means performance-tuned models. Many of us may not like tuner cars, but considering Time Attack and drifting, don't you want to race machines made to take on the challenge of stuff like Tsukuba and all? I do believe that if you don't see a tuner that you like, you can always make your own. Tuner cars are as much a part of our worldwide automotive culture as Kei cars and sports cars. Neon lighting is highly debatable. Surely they do no performance enhancement, but you'll see some lighting on the Blitz Skyline in GT4 when the lights are turned on. It would be interesting to have some neons. Not so much tuner-style, but can you recall some times with the 24 Hours at the Nürburgring when some cars have laser shows and stuff from their cars?
I don't believe the GT series is leaning towards anything resembling NFS: Underground because tuners have been around since the first game. Remember the Mazda Demio A-Spec in GT1? Maybe not a powerful or significant tuner, but there have been high-performance models in the series since game one. Since GT3, I think if you have completely modded up a car and can rule any race track, one should be able to modify the aesthetics of a car so that it looks the part of a pure racer rather than just a glorified sleeper. If I tuned an Aston Martin DB9 to be like a GT race car, I want that Aston Martin to look like a performer in addition to being a performer. If PD wanted to make every car look the part of a racing champion, every car would have to be continually looked over to see what kinds of performance looks can be made so that a mild-mannered street car can be converted into a pure race car.