You don't need the data. Hire UX experts, set up focus groups, dogfood your team, read bug reports from your users. Anything else is an invasion of their privacy and a sign of disrespect for the customer.

There are exceptions, though. If you can improve safety of your product by in-depth analysis (cf. FADEC [0]), and the customer is on board (no pun intended), go for it.

[0] https://en.wikipedia.org/wiki/FADEC

It isn't. Provided you got consent, defined the scope in which the data will be used and is held accountable to not change the scope without clearly asking for consent again.

This also requires the user to trust you will act within the scope defined, and that can be hard to verify. From a user perspective it's generally safer to just not grant consent.

If you are improving it in the interest of the customer, there is nothing wrong. However, if you are using it in a way that not only does not benefit them, but harms their wellbeing, then people won't like you. Additionally, you might sell their data to third parties and breach the confidence that the customers have in your service.