Is this a good layout for a 5A buck converter?
No, it's not a good layout. Star pointing ground returns is a good idea but I'd use a double sided PCB for best layout. The circuit doesn't help you get the best layout either. Pin 8 of U1 should ideally directly connect to the grounds on the big smoothing capacitors and take no current up any of its length that would flow into those three big caps. U1 has to "measure" the output voltage via a potential divider and it needs a stable 0V reference to do this.

Incidentally, the circuit diagram does not match the layout - where are the feedback components R5 and R6? Where is CR1? I've already worked out that C9 = C10/11/12 but you are doing things that I can't see on the circuit. This is bad dude. OK I can see that CR1 has become D1 but this needs to be right up at Q1 to work effectively unless you don't mind replacing Q1 now and then and interfering with your neighbors AM radio.

In short, you may get away with 1 sided PCB but more work needs to be done.

Please post a circuit that matches the artwork as well - we are not paid to help so don't make life difficult for us.