In my opinion, it's not bad. It's the USE of it that can be bad. In some point or other, self-insertion always happen at least once in the entertainment, so it's unavoidable anyway.
If the self-insert is simply a character that represents 100% (the goods and the bads) the author, or presents good personality, or even both, then it can be good self-insert. However, if the self-insert is basically Mary, or basically a complete failure, then it can be boring, tedious and no fun, and thus, bad. What
>>35357359 said is correct
In summary, there's nothing wrong with self-inserting, especially on entertainment that officially presents avatars, as long it's good and not serious.
