You can change database credentials any time if it seems useful to you. You have full root on the mysql database. So ddev mysql -uroot -proot
will get you in there where you can GRANT ALL on db.* to 'mynewuser'@'%' IDENTIFIED BY 'mynewpassword';
That will create a new user with full privs on the 'db' database. But you can also create new databases and new users as much as you want. And if you want, you can change the password of the 'db' user. We try to make it as simple as possible for local dev... but if you want to make it more complicated, go for it!
There are a couple of items in the FAQ that may help as well - about multiple databases and database credentials.
If you remove the default credentials, of coures, you have to change your CMS's settings files to credentials that will work.