Browserstack

iOS

Mockzilla should work out of the box on iOS with Browserstack.

Android

Browserstack seems to proxy local traffic by default. In your client app you'll need to bypass any proxy.

OkHttp Example

OkHttpClient.Builder()
    .proxy(Proxy.NO_PROXY)
    .protocols(listOf(Protocol.HTTP_1_1)).build()

Ktor Example:

See demo app example here.