Bad for the dollar, bad for America.
Any deal with Iran, should mandate their use of the dollar.
Too late now.
Actually it is excellent t for the US and bad for Iran. The US is the least evil currency, for now, so Iran can expect a lower return on its oil as the Euro slides to irrelevancy. It means in dollar terms Iranian oil will be cheaper than it needed to be. Less money for Iran more for US.
My guess is that the Iranians are hoping to dominate Europe. Having Euros will make it easier to finance the immivasion.