Correct. Only the root account on most Linux-based systems can connect to MariaDB (and MySQL) as the root account. This is to prevent poor authentication practices as have been witnessed time and again where a WordPress (or any other CMS) installation uses the root database account, gets compromised, and gives the attackers full control over the database and ...
(Since the question was resurrected, I guess I'll add my 2 cents.)
I would verify, if the username conforms to the DB2 naming rules. Since you used authentication plugin previously, I suspect you could ignore those naming rules. Now that you created the user on the server directly, you need to follow them. Here's the link to the rules:
a simple answer - user db2smth need the 'connect' privilege --
you can try :
db2 connect to dbname user db2inst1
GRANT CONNECT ON DATABASE TO USER db2smth
How do I grant all privileges to local db2admin with DB2 9.7 on Windows?