Gwentdb is the wrong place to look since anyone can post any meme deck and bias the results. Instead you should look at Gwentup's meta reports, in particular the 4250+ section.
Brouver has five more common golds: Ithlinne, Iorveth: Meditation, Aglais, Isengrim: Outlaw, and Schirru. Ciri: Nova is at 18.2% popularity.
Eredin has three more common golds: Caretaker, Caranthir and Ge'els. Ciri: Nova is at 48% popularity.
Emhyr only has 1.6% usage of Ciri: Nova.
Voorhis only has 5.1% usage of Ciri: Nova.
Jan Calveit has 5.9% usage of Ciri: Nova.
Foltest has three more common golds: John Natalis, Shani and Djikstra. Ciri: Nova has 61.8% popularity.
Crach has one more popular gold: Hym. Ciri Nova is second with 84.1% popularity.
Harald only has 2.8% usage of Ciri: Nova.
Eithne has six more common golds: Aglais, Isengrim: Outlaw, Iorveth: Meditation, Ithlinne, Schirru, and Muzzle. Ciri: Nova is at 16.7% popularity.
Clearly the card isn't ubiquitous. Some decks (and entire factions for that matter) cannot utilize her well. With other leaders (Eredin, Foltest, Crach especially) it's as though they need Ciri: Nova to be competitive.
Also I doubt she's the most common neutral card. Looking at the meta report, Summoning Circle is probably more common. No surprises there and that's completely a consequence of how important spies are.
Brouver has five more common golds: Ithlinne, Iorveth: Meditation, Aglais, Isengrim: Outlaw, and Schirru. Ciri: Nova is at 18.2% popularity.
Eredin has three more common golds: Caretaker, Caranthir and Ge'els. Ciri: Nova is at 48% popularity.
Emhyr only has 1.6% usage of Ciri: Nova.
Voorhis only has 5.1% usage of Ciri: Nova.
Jan Calveit has 5.9% usage of Ciri: Nova.
Foltest has three more common golds: John Natalis, Shani and Djikstra. Ciri: Nova has 61.8% popularity.
Crach has one more popular gold: Hym. Ciri Nova is second with 84.1% popularity.
Harald only has 2.8% usage of Ciri: Nova.
Eithne has six more common golds: Aglais, Isengrim: Outlaw, Iorveth: Meditation, Ithlinne, Schirru, and Muzzle. Ciri: Nova is at 16.7% popularity.
Clearly the card isn't ubiquitous. Some decks (and entire factions for that matter) cannot utilize her well. With other leaders (Eredin, Foltest, Crach especially) it's as though they need Ciri: Nova to be competitive.
Also I doubt she's the most common neutral card. Looking at the meta report, Summoning Circle is probably more common. No surprises there and that's completely a consequence of how important spies are.