For one, FEMA needs more disaster relief funds. There are members of Congress who will vote against it because they're dumb enough to fall for Trump's lies and will vote against it because they think FEMA will spend it on illegal aliens.
For another, some people who have internet access will not reach out to FEMA for needed help because Trump convinced them it's pointless.
For another it reduces FEMA's morale.
For another, he's divisive during a disaster when we should be united.
There's nothing useful during a disaster by misleading people with lies.