Personally, while I acknowledge that a true Muppets vehicle would be more spray paint than fine finish, the technical incongruity doesn’t bother me much here for a few reasons.
First, while the interior story of G-Force is now that Grosse owns it, the art deco surroundings still have to be taken into consideration. This isn’t a dedicated Muppets land; it’s a Muppets ride that exists within Sunset to a certain extent. Same idea behind the clock tower outside DL Peter Pan being more Bavarian than Big Ben, for instance.
Second, I feel like there should be a connection between the finish of the car and the finish of the guitar since they’re supposed to visually read as part of the same “prop”.
Third, even if you put all that aside and insist that it has to be in-universe to the same degree one would expect of a themed land, it’s kind of open-ended as to whether it’s supposed to be a genuine Muppets vehicle, part of the decor of G-Force, or some sort of re-purposed prop from their album cover.