Tertullian, Against Marcion 5.11
If "god" has become a generic label through the fault of human error, to the point that many gods are named and believed in throughout the world, the blessed God of our Lord Jesus Christ is still understood to be none other than the Creator—who both blessed all things (Genesis) and is blessed by all things (Daniel).
So even if "father" can be applied to a sterile god—with no title more distinctive than that of Creator—the Father of mercies will be the very same one who is called merciful, a mercifier, and most full of mercies.
You have an example of his mercies in Jonah: the mercy he granted the Ninevites when they prayed (Jonah); how easily he was moved by Hezekiah's tears; how he forgave Ahab, Jezebel's husband, the blood of Naboth when Ahab begged for it; how he immediately granted indulgence to David when David acknowledged his fault—plainly preferring the sinner's repentance to death, certainly out of an impulse of mercy.
If Marcion's god has ever produced or promulgated anything of that kind, I will acknowledge him as "the Father of mercies."
Latin
Si 'deus' commune uocabulum factum est uitio erroris humani, quatenus plures dei dicuntur atque creduntur in saeculo, benedictus tamen deus domini nostri Iesu Christi non alius quam creator intellegetur, qui et uniuersa benedixit — habes Genesim — et ab uniuersis benedicitur — habes Danihelem —; proinde si 'pater* potest dici sterilis deus, [nullius magis nomine quam creatoris] misericordiarum tamen pater idem erit, qui misericors et miserator et misericordiae plurimus dictus est. habes apud Ionam cum ipso misericordiae exemplum, quam Niniuitis exorantibus praestitit, facilis et Ezechiae fletibus flecti, et Achab, marito Iezabel, deprecanti sanguinem ignoscere Nabuthae, et Dauid agnoscenti delictum statim indulgens, malens scilicet paenitentiam peccatoris quam mortem, utique ex misericordiae adfectu. si quid tale Marcionis deus edidit uel edixit, agnoscam 'patrem misericordiarum'.