I don't see any evidence that Harley Quinn would be to blame if this movie underperforms.
Again, as you and I have said, it was the marketing. A Harley Quinn movie could work now or whenever. AS you point out, Aquaman was introduced in the trainwreck called JL and his solo movie went on to be one of the highest grossing DC movies. So I don't think it was SS either.
I just really think
1) The full marketing push started way too late
2) The actual marketing (even the title of the movie) wasn't good