Absolutely not. I don't think it's in any of our interests to ever encourage microtransaction-style selling of digital assets like that lest it turn into a Sims 3/Sims 4 EA cash grab mobile game. It kills all games that utilise this and turns them into something very different, and it would absolutely kill the Planet games.
It is always, always better to have full-packaged themed DLCs made with love, rather than having $1 for a monorail, $1 for a boat, $1 for a wall set, $1 for a path texture...
Even if you only want a few things from a given DLC, it's certainly better for the game and its quality as a whole to have them as full DLCs.