in general, the mutation/skin guidelines go;
-do not make a class resemble another existing class, especially if their sizes match
-modifications/skins heavily referenced or straight up rip-offs from other media are not allowed, e.g jurassic park rex mutation/skin. these will be removed from workshop
-do not remove crests, or other obvious designs on the animal. e.g, cerato's nasal horn, oviraptor's crest, the crests of dilo and cryo

that ought to sum up the most important ones. this guide gives a nice list for each class:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2263759090

sadly we still lack a official mutation guidelines list, put together by the devs. in the meanwhile, following these rules should be enough.

on the skin side, just be original and don't steal designs. that is about all on 'skin' guidelines.