Can we change the default owner of mongodb to other than 'mongod' user?
Yes! In /usr/lib/systemd/system/mongod.service. Changing User parameter value to wanted user name. Remember to change Group too and then chown -R all files and directories to that new user.

[Unit]
Description=High-performance, schema-free document-oriented database
After=network.target
Documentation=https://docs.mongodb.org/manual

[Service]
User=mongod
Group=mongod
Environment="OPTIONS=--quiet -f /etc/mongod.conf"
ExecStart=/usr/bin/mongod $OPTIONS run
ExecStartPre=/usr/bin/mkdir -p /var/run/mongodb
ExecStartPre=/usr/bin/chown mongod:mongod /var/run/mongodb
ExecStartPre=/usr/bin/chmod 0755 /var/run/mongodb
PermissionsStartOnly=true
PIDFile=/var/run/mongodb/mongod.pid
# file size
LimitFSIZE=infinity
# cpu time
LimitCPU=infinity
# virtual memory size
LimitAS=infinity
# open files
LimitNOFILE=64000
# processes/threads
LimitNPROC=64000
# total threads (user+kernel)
TasksMax=infinity
TasksAccounting=false
# Recommended limits for for mongod as specified in
# http://docs.mongodb.org/manual/reference/ulimit/#recommended-settings

[Install]
WantedBy=multi-user.target