How can Ionic be setup to render in the iOS style by default?

in index.html just define mode

For iOS

<html lang="en" mode="ios">

For android

<html lang="en" mode="md">

You just need to do this:

app.module.ts

imports: [
IonicModule.forRoot(MyApp,{
    mode: 'ios'
})
],

Note: From @sebaferreras

Btw, by setting this config the app will use the ios styles and components even if you run it on an Android device. Please notice that ?ionicplatform=ios is just used to see how the app looks like when using the browser, but does not affect how the app is built. But setting mode: 'ios' will force the ios mode and apply the ios styles even if you build and create a .apk file.