MISSING!! Besides the disabled=yes error, where is the interface? designated on that rule.
Try
/ip firewall mangle
add action=change-mss chain=forward comment=“Clamp MSS to PMTU for Outgoing packets” new-mss=clamp-to-pmtu out-interface=wireguard1 passthrough=yes protocol=tcp tcp-flags=syn
If you dont see any difference with that mangle rule you an also try this mangle rule.
/ip firewall mangle
add action=change-mss chain=forward new-mss=1380 out-interface=wireguard1 protocol=tcp tcp-flags=syn tcp-mss=1381-65535