Linux Mint comes by default with connectivity options enabled, giving the best out-of-the-box experience on the local network
to new users by allowing remote desktop support, connecting to network printers, sharing files with others and more. Xhost + is part of those connectivity options. Your Internet router's firewall will block any attempt from outside the local network (i.e., the Internet) to reach your computer. So even with this service running, unless you specifically configure your Internet router's firewall to allow traffic from the Internet to go to this service on your computer, you are not at risk for remote attackers. This goes for all connectivity options (see here viewtopic.php?f=18&t=106417&p=600177#p600177
There is only a possible security issue with running the X server without access control on the local network. If you are on a public local network (like a WiFi hotspot somewhere outside your home), I recommend enabling Linux Mint's firewall, and setting it to default deny incoming traffic. This will protect you on the local network also, unless you specifically configure a port on your computer to be reachable by others in that firewall.