What is your Mavproxy load / command line. Please post it.
I assume your PI is connected to WiFi and on the same network as your computer.
Can you PING the PI from the PC?
You will need two MAVPROXY start-up options.
One for the PI to connect to the AP via serial (Looks as if this is done as you have it working)
One for Mav OUT from the PI over either TCP or UDP
If a serial port is specified, an optional comma-separated baud rate may also be specified. If present, this overrides the rate given by --baudrate.
mavproxy.py --master=udp:192.168.1.1:14550 --master=/dev/ttyUSB0
If connecting to a remote IP address, the udpout or tcpout arguments should be used:
Forward the MAVLink packets to a remote device (serial, USB or network address/port). Useful if using multiple ground station computers or relaying the stream through an intermediate node.
MAVlink usually uses port 14550 for IP-based packet forwarding, though other ports can be used if needed.