The problem here is that such a terrorist attack was caused by anti-muslim rhetoric. If more immigrants came here, especially refugees, and started committing a ton of crime like they have in Europe (Germany, Britain, France, etc.) the negative view towards muslims will only increase. It would be better to disallow immigrants from said countries for a short time to help these problems be forgotten in the public eye so that terrorist attacks from both sides will have a less likely chance of occuring.
The attack on the mosque in Quebec is, if anything, in contrast with the Trump Administration's policies. The attack was on a mosque, victimizing the Muslim community in a way that Trump has repeatedly ignored. Furthermore, the Muslim community has been attacked by the Trump administration multiple times, including through the recent Muslim ban, and to use these victims of gun violence and hate crimes as a political tool is inappropriate.
The shooter at the mosque in Quebec was undoubtedly inspired by the Islamophobic rhetoric that has been building to a fever pitch, even before the run up to last years game changing US election. The fact that the White House would see to use this terrorist attack as a justification for their racist immigration policies frankly defies anything that even resembles logic.
How in the world can a Caucasian man attacking a mosque justify a ban on travelers from seven Muslim countries? Is the White House trying to say that there would be no attacks of any kind if there were no Muslims at all? I wish they would just come out and say exactly what they mean instead of presenting one alternative fact after another.
Donald Trump spent much of his time on the campaign trail demonizing Muslims and provoking the kind of fear and hatred that likely inspired the attacker. It is wildly inappropriate for him to use the victims of this attack to support his policies, which have already included banning people from several Muslim majority countries from entering the USA.