Yes it is. The ECB implements the monetary policy of the euro area. It pursues its mandate of price stability with the instruments defined in the Treaties. Outright purchases of marketable instruments are explicitly mentioned as a monetary policy instrument (in Article 18.1 of the Statute of the ESCB). This includes the possibility to purchase instruments such as government bonds, as long as they are bought on the secondary market from investors and not on the primary market, i.e. directly from Member States.