I think it is.

Everything we know about Jill and the world is enough for me to believe she's Shiva's dominant.

She's from a fallen kingdom in the north, and if this world is set in the northern hemisphere, that likely means cold/snowy climate. All of the kingdoms currently with blacked out eikons either refer to a male dominant or with gender neutral language that refers to a champion who is established as a battlefield combatant, which Jill explicitly is not.

Each nation has its own dominant, and one would assume that includes the northern territories before their fall to Rosaria. Jill is the only character we know of from that region.

with all of this and the artwork that looks like Jill, I can only assume its post-timeskip Jill serving as Shiva's dominant.