This question is probably the most asked question in many groups. The answer, short and sweet...YES!
An editor to a serious writer is necessary.
But, let's talk about when you need an editor. What type of publishing are you looking to do? An Indie author or self-published author will need at a minimum a copyeditor before ever thinking about publishing. You are the writer and no matter how many English degrees you have or how many times you review your work, You will always miss something.
Oh, but your mom is an English teacher. That's great, but she is not an editor. They are not one in the same. Sure, she will be extremely helpful, but you will still need an editor.
Maybe you are looking to be traditionally published. You still need an editor. Most traditional publishing houses have their own editors, so you do not have to worry. But, if you are looking to get noticed by one of those companies, you want your submission to be clean and error-free.
Would you show up to a job interview in wrinkled and smelly clothes? Then why would you submit a manuscript full of errors? Tells people that you don't care about your work or how you look.