I’d say the best modding experience is when there’s a documented API. You kind of don’t want to look at the code behind that documented API, because you don’t want to rely on behaviour that can change at any point.
So, if there is a modding API and it has all the features that you need for the mod you’re looking to create, then I’d say it doesn’t make a difference.
If there is no modding API, then as others already explained, you can fork said open-source project, i.e. you don’t even need to glue a mod at the side of it, you just change the game itself to work like you want it. In this scenario, open-source makes it significantly easier to modify a game.
I find that question difficult.
I’d say the best modding experience is when there’s a documented API. You kind of don’t want to look at the code behind that documented API, because you don’t want to rely on behaviour that can change at any point.
So, if there is a modding API and it has all the features that you need for the mod you’re looking to create, then I’d say it doesn’t make a difference.
If there is no modding API, then as others already explained, you can fork said open-source project, i.e. you don’t even need to glue a mod at the side of it, you just change the game itself to work like you want it. In this scenario, open-source makes it significantly easier to modify a game.