Technically there is one but I don't think it will fit your expectations of a skirmish mode.
There is a training option when in camp mode but you are only fighting against another meve(the character you are playing as) using the exact same deck as yours.
Better than nothing I guess.
In Gwent you can only play against the Monsters AI which uses the same deck every time.
This way at least I can mix it up.
A shame they didn't add skirmish with randomly chosen pre-made decks in Thronebreaker.
I own the game but haven't played much of it yet.