The Jews were given a day to mourn all of our tragedies. That is Tisha B’Av. Also, setting it in the month of Nissan probably wasn’t a good idea, as we’re not supposed to mourn during Nissan.
While I don’t observe Yom HaShoah, I think it’s completely disrespectful of the people in Israel who ignore things like the siren. Everyone else is standing still. The country comes to a halt for 2 minutes. Is it so hard for you to do that also? Even if you don’t agree, don’t openly ignore it. You live in a country that made this a day of remembrance. At least don’t openly go against it.