Yes. Modification/Altering is different from addition. You can display your own stuff over the game window, same thing WTRI is doing or any other game overlay app (Discord, Steam, MSI Afterburner, RivaTuner OSD, etc.).
If there is no modification and only playing with the already available stuff, then it’s fine. There is my own app called WTbgA that uses the same LAN API. No modifications, but only processing the API’s data.
Just know that processing the information on the game’s screen can be considered cheating, it’s best sticking to the LAN API (127.0.0.1:8111). The API is shared over the local network too, that’s why I’m calling it LAN API and not just local API. However, you can disable accessing the API via firewall rules.
You can always make a suggestion Suggestions - War Thunder — official forum
I already had a suggestion to include network metrics in the API, you can make one too :)