Yes, SP1 is unnecessary, because service packs are cumulative, meaning that they roll-up all fixes that came before them into a single package.

Service Pack 2 was released on August 6, 2004 with a special emphasis on security. Unlike the previous service packs, SP2 adds new functionality to Windows XP, like an enhanced firewall, improved Wi-Fi support, WPA encryption compatibility, a pop-up ad blocker for Internet Explorer, Bluetooth support.