BLE beacon 2.4GHz PCB antenna layout for SoC Nordic nRF52832
Put on the solder mask.  Metal being directly exposed only brings heartburn.
Soldermask (unless made of some weird material) generally has very little effect on 2.4GHz RF.  You're likely to have much worse loss from that far-too-simple matching network.
Note that the (old?) Nordic RF beacons (also round) put soldermask over their antennas (which were just a single trace--aka quarter wave whip).