It's terrible in the biggest way you can fuck up your UI: doing the opposite of what every user expects
Billions of people are all used to typing into masked (or unechoed) password fields, having done so in a wide range of UIs a shitload of times. Typing into a password field and seeing it reflected back feels like a slap in the face, the security context is irrelevant, it's just a massive discourtesy.
Your website is not going to be the very first thing the user has ever logged into. It is not your place to try to reinvent such fundamental things, changing this on your site is on the same level of jackass hubris as reversing the tab order or mouse coordinate system with javascript. All you're going to do is infuriate people.
Billions of people are all used to typing into masked (or unechoed) password fields, having done so in a wide range of UIs a shitload of times. Typing into a password field and seeing it reflected back feels like a slap in the face, the security context is irrelevant, it's just a massive discourtesy.
Your website is not going to be the very first thing the user has ever logged into. It is not your place to try to reinvent such fundamental things, changing this on your site is on the same level of jackass hubris as reversing the tab order or mouse coordinate system with javascript. All you're going to do is infuriate people.