Opinions on OCs and ACC's and Self Inserts vary by a huge amount, partially depending on whether talking to a author or reader, and partially depending on use.
A OC or ACC can be a good thing, if done carefully, and if really needed. The issue is a lot are either not needed, or implemented badly, author avatars, not likeable, or turn into MarySu/GaryStu's
I've seen a few over the years done well, and the stories in them are quite enjoyable reads.
The formula to good ones is simple in concept, but hard in practice.
1. Do not make them all powerful/all knowledgeable and solve or have answers to all canon issues near immediately and can do no wrong. Even if have knowledge or power, give them flaws.
2. Do not make them be little more than a author vent or lecturer to all that the canon cast does wrong
3. Make them have a full background with a not so perfect past (make them have character flaws) that not only appeals to canon characters, but to readers.
4. They must serve a relevant purpose. That purpose can vary, either to train/teach the cast something they otherwise would not know, or to tell a story from a non-canon view. Be saved by one or more canon cast members, or alter canon history, perhaps just by their presence or knowledge.
Examples of ones I personally liked over the years:
Jeffery Kevin Davis - First appeared in the multicrossover NGE/RANMA/Other story of 'Sic Semper Morituri', and in other stories by same author.
Captain Andrew Simms - from a whole bunch of FMP fan fiction stories by Steve Edwards, assumes a 2nd sub is fielded by Mithril, by a more traditional and seasoned commander that is still fairly young.
Kyle, from a series of Ranma stories by Enigmatic Magus (he went by a different pen name years ago, but has been that one for awhile). Kyle is a normal guy whom has to jump universes, after meeting Mara (Ah/Oh My Goddess), Makoto (Sailor Moon), and Nabiki (Ranma 1/2).
Pretty sure that last served as the basis for another ACC made later for a two arc story called 'Sleeping with the Girls' by Admiral-Tigerclaw, which is also pretty good.