Yes, it is completely legal to use a VPN to watch Netflix. However, depending on your usage, it may be against Netflix’s terms and conditions to use a VPN to watch geo-restricted content.
If Netflix detects that you are using a VPN, you will likely receive an error message and you won’t be able to access that Netflix content until you turn off your VPN.
While Netflix blocks a lot of VPNs, there are many good VPNs that are able to access Netflix, like ExpressVPN and and Private Internet Access.