Nair is good if youre at the end of the stage, especially if you think theyll di for an upsmash.
If a Falco is bold enough to come up to you and try and grab you first (and a lot of good falcos will try), dtilt puts you at the advantage of achieving the grab first, quite honestly its the only thing that saves you from being ownd by his cg each stock. However if he is not trying to go for the grab and you are, most likely you are going to get owned and grabbed yourself. It's ok though, chaingrab aside pikachu has some good tools for fighting falco, and most likely theyll be playing scared and end up taking a lot of damage because of it. And then generally over the course of the fight theyll probably fall into the grab when their options are limited. Just keep an eye out for their attempts at grabbing you and keep that dtilt ready, and make sure it isnt terribly predictable or itll just get powershielded.