You could donate to the project but I don't think a mirror would help.
Mirrors still see a specific instance as the truth and sync to that. A mirror would make it possible for someone to download an update from you while the truth instance is down (or always from you if you're closer and faster) but without that first truth instance there'll be no new updates. There won't be syncing across mirrors. And it wouldn't really impact any DDOS because it's still that first instance being targeted.
DDOS mitigation can take one of two main ways. Add enough server power to overcome the DDOS, this is insanely expensive and out of each for most all but like Amazon and Meta. The second is to cut off whatever is doing the DDOS, either by disabling the specifically protocol or endpoint being used or not allowing certain IPs to connect. These are probably what they are doing already and there's not much you can do as an outsider to help with this either sadly.

It's a good habit when if you're hitting something weird and it just doesn't work to check a status page. It's saved me so much time over the years. Here it is for Arch AUR