"nothing to do with Meuller" is simply not true. You don't have to guess here, Trump is very open in communicating his mistaken belief that memo would kill the investigation. As such, it doesn't really matter what it is, trying to address it is tainted by corrupt intent.
Sure, FISA shouldn't exist. I hope you understand that Trump isn't trying to clamp down on FISA.
Trump's opinion on the memo does not carry the force of law, but remains his Constitutionally protected right of freedom of expression, just the same as you enjoy. If he chooses to act in an official capacity on that, he would have to comply with applicable laws. Until then, the President can use the Bully Pulpit as he sees fit.
Trump shouldn't need to clamp down on FISA, but he should ensure that criminal laws related to abuse of the FISA process are prosecuted to the full extent of the law, and based on the memo, the abuses appear to be many at the very highest positions of the FBI and the Justice Department.