Technically, you can edit whatever you want without repercussion as long as it's kept private.
It only becomes a potential problem when you release it to the public.
There isn't really anyone policing the content, and checking in every case whether the person asked permission or not, it's just assumed by default in many cases that the person did, and in some cases the original work may not even be known and be assumed to be original from whoever released it last.
A person only becomes a target of attention in the subject of permissions when an alleged violation of copyright comes to light, namely editing something without permission.
And, quite honestly, if you're "only" editing skins, or editing maps that no one really gives a damn about, it's generally not a problem: no one is going after you for providing an edit of a GodZ MH map for example, or if they do, it's not over the fact you edited it, but it would be mostly because no one wants more GodZ maps.
But it's a whole different thing when it's known work, like known maps, or any other work from known creator, especially those known for holding their rights over their own work closely, and not allowing anyone to edit it by default.
If you do edit any of their work without their permission, and release it to the public, that's when you get into trouble.
And before anyone misinterprets what this actually means: it's NOT that only known creators get the privilege of having their work protected, or that their work is more important.
It's more that these creators (map and mod makers) often take the time to add a license (ReadMe) to what they created, to make it clear what other people can do with it and how, meaning they do give a damn about how their work is handled and distributed, and even edited, so it must be respected as such, whereas others not so much given that in the case of a map or anything else that has absolutely no license or ReadMe file of any kind, then I think it's acceptable to consider it "free real estate" and do whatever you want to with it, even though the courtesy of trying to reach out to the original creator in these cases is still highly appreciated nonetheless.