Is using Joi for validation on top of Mongoose good practice?
It is a common practice to implement a validation service even if you have mongoose schema. As you stated yourself it will return an validation error before any login is executed on the data. so, it will definitely save some time in that case.Moreover, you get better validation control with joi. But, it highly depends upon your requirement also because it will increase the extra code you have to write which can be avoided without making much difference to the end result.
IMO, I don't think there's a definite answer to this question. Like what @omer said in the comment section above, Mongoose is powerful enough to stand its own ground.
But if your code's logic/operations after receiving input is pretty heavy and expensive, it won't hurt adding an extra protection in the API layer to prevent your heavy code from running.
Edit: I just found this good answer by a respectable person.