Try
mkdir -p /var/log/mosquitto /opt/zram/log.bind/mosquitto
chown mosquitto /var/log/mosquitto /opt/zram/log.bind/mosquitto
systemctl start mosquitto.service
Create /lib/systemd/system/mosquitto.service.d/override.conf to contain
[Unit]
After=network-online.target zram-config.service
Wants=network-online.target zram-config.service