Recently Japan is in the midst of a "camping boom", and car camping is quite popular. Whenever I visit highway rest stops in the early morning, there always seems to be at least one person/group sleeping in their car (easily identified by the covers they put over their windows for privacy and insulation). And a nearby campsite is always hopping on the weekends, during the temperate months at least. So I would say the general attitude is quite supportive in terms of infrastructure, laws, etc.
But yeah if folks are not well behaved, especially with regards to properly disposing of trash and human waste, it would not be well received.
Edit: For fun I just googled "キャンプブーム" (camp boom) and found this article calling it annoying, and complaining specifically about people with bad manners not keeping campsites clean, being noisy at night, and starting fires where they are prohibited [1]. Note this is more from the perspective of veteran campers complaining about newbs, and more directed at Japanese people rather than foreign visitors. But yeah maybe you are on to something.
But yeah if folks are not well behaved, especially with regards to properly disposing of trash and human waste, it would not be well received.
Edit: For fun I just googled "キャンプブーム" (camp boom) and found this article calling it annoying, and complaining specifically about people with bad manners not keeping campsites clean, being noisy at night, and starting fires where they are prohibited [1]. Note this is more from the perspective of veteran campers complaining about newbs, and more directed at Japanese people rather than foreign visitors. But yeah maybe you are on to something.
[1] https://ramunecamp.com/campboom-uzai/