I want to make a game and I see some paid game is open source, Is it good to make paid game to open source?
Good for what? Good for your sales? Most likely not. People can get the game for free now, after all! (Given they’re tech-savvy enough to build the game themselves). That being said, if the move to open source gives you enough media attention, then yeah, you might be able to turn the situation into an overall profit. More likely than not, games will get open sourced because their creators want to, though. It gives other game makers the chance to learn from your code, and increases the game’s potential longevity, since others can pick up where you left if you ever (have to) abandon the project.
In terms, of a business model: it exists in the market. Software like Asesprite is sold as commercial software on storefronts like Steam and Itch, but is also available to be compiled from source. Most people who are willing to pay are willing to pay for convenience, and if your game is quality enough, people will pay for it.
In terms of code quality: depending on the license you’ve chosen for opening the source code, you’re likely to receive the benefits of being open-source, like improved security, community, and feature contributions to source.
However, as the esteemed @njamster has said, your game is now free. Nothing is stopping a competitor from distributing binaries instead of your storefront, or even taking your source and improving upon it to create a better product. None of these things can be taken for certain or for granted, and as such, like every monetization strategy, there are both benefits and drawbacks to doing so.