Making a character based on you is not a terrible idea. after all we're all individuals with some character to us right?
The problems start when you try forcibly enhancing "yourself" (because we're all cool and special right?) because you want to be "cool" and kinda ending up like sonic the hedgehog.
Trying too hard and ending up as the polar opposite of cool with a mary sue asswipe of a character.
A trend I've seen a lot is something sort of like this, but a byproduct of the author's utterly deflated self-esteem and an attempt to make them seem like a worse person than they actually are. They make a character who is them, but has some kind of element to them that shoots them into "awful" territory (the most common one I've seen is where it's them but they go out of their way to verbally lash out at people and make snide comments to peoples' faces.)
When this isn't just the author trying to make themselves seem like a worse person than they actually are, it's usually a specific attempt to not create a Mary Sue (and in the process, creating one who's just as bad--it's still an idealized version of the author, it's just not one other people like.) These characters are arguably even more common than traditional Mary Sues (in fact, the archetypal Sue, Ebony "Enoby" "TaEbory" Dark'ness Dementia
Raven TARA Way nut Mary Su OK, has elements of this; the author repeatedly makes claims the character isn't a Sue because they're, variously, depressed, suicidal, or has a tragic backstory that's never expanded upon, but because the author glorifies these things, it arguably makes the character even more of a Sue than if they weren't.)
The delicate balance of creating a self-insert isn't worth it, really; you have to have quite a bit of self-control to stop the character from straying into either Sue or Anti-Sue territory, and for what? Writing yourself? Is that honestly worth taking such painstaking measures to prevent it from becoming a power fantasy? The one place I'd say it's worth it is if you're deliberately trying to poke fun at self-inserts.