Why is Tommy shooting at the fourth wall?

This scene is actually a direct reference to the scene from The Great Train Robbery where a similar scene concludes the movie.

Here in this below video at 0:25 secs Martin Scorsese himself mentions the very film and says "...that's why I had Joe Pesci shoot at the camera"

Youtube Link

You can also find the mention of the same connection in here

In an interview with the American Film Institute, Scorsese explains the connection between his movie and Porter's: "Basically, in Goodfellas, it's a bunch of outlaws who do this incredible robbery. And then they all kill each other, and the police get them at the end. It's exactly the same story." In other words, Scorsese ripped off the scene. But, you know what they say—imitation is the sincerest form of flattery.

Here, by "Porter" he means Edwin S. Porter

That shot is a reference to the final shot of the 1903 silent film The Great Train Robbery, which ends with one of the outlaws shooting at the camera.

See the comparison here on YouTube

His past will always follow him. He’ll always be watching his back. Don’t mistake him for an average nobody, because he once lived the life of a violent criminal, aside men who wouldn’t hesitate to kill. It’s also an homage to an old western movie, when a cowboy shoots directly at the camera.