I think it was Bourdain's book that gave people that no-fish-on-Monday idea.
I can't imagine there would be a problem ordering fish on a Monday at any upscale restaurant. For example, Le Bernardin, the temple of haute fish cuisine, is closed on Sunday, but open on Monday. And I don't think they're wanting for customers on Mondays.
Also, I would not hesitate to order fish no matter what the day at neighborhood restaurants we frequent and trust.
Frankly, I think the question of bad fish is more about how a restaurant handles it rather than whether fish markets are open or closed.
Edited to add: And what about all the best sushi places? They're not closed on Mondays.