Any mods would have to be authorized by the app developer, and those mods must then be within the Apple App Store restrictions around apps and app mods.
Permissible mods might include new levels or an associated level editor, new skins, and other possibilities. That does not include modified code or adding new code within the app.
If you want the ability to add permissible mods, ask the app developer to add support for that.
Complying with all of section 1, and section 2.5.2 are the main requirements and restrictions around mods:
As for loading or side-loading who-knows-what from who-knows-where, that can be benign, or can be malware, and that’ll get blocked by iOS security short of jailbreaking, or beside legally-permissible side-loading. And side-loading won’t allow access or mods outside the current app sandbox, so side-loading mods for unrelated apps isn’t happening.
Which is a lot of words for “no”.