Successful mitmproxy-3.7 setup tested on OS X 10.13.6 and iPhone X running 12.1.4
Enable IP forwarding and disable ICMP redirects to keep the iPad sending traffic to the proxy
sudo sysctl -w net.inet.ip.forwarding=1
sudo sysctl -w net.inet.ip.redirect=0
net.inet.ip.forwarding
Enable IP forwarding between interfaces