First you need one png file as your launcher icon.

Then you need to resize this icon file to needed resolutions.
There is a simple service to resize your app icon to all needed resolutions with rocket speed.
Then you can download all your needed icons.

Then replace each ic_launcher.png via image with correct resolution.
webrowser/android$ find -name "ic_launcher.png"
./app/src/main/res/mipmap-hdpi/ic_launcher.png
./app/src/main/res/mipmap-mdpi/ic_launcher.png
./app/src/main/res/mipmap-xhdpi/ic_launcher.png
./app/src/main/res/mipmap-xxhdpi/ic_launcher.png

Or you can use Image Asset Studio to create App Icons.
1.open Android Studio poject in your react native application.

2.use Image Asset Studio

In the Foreground Layer tab, select an Asset Type, and then specify the asset in the field underneath:
Select Image to specify the path for an image file.

If your app supports Android 8.0, create adaptive and legacy launcher icons.

If your app supports versions no higher than Android 7.1, create a legacy launcher icon only.

Check the result
react-native run-android

Refer

https://developer.android.com/studio/write/image-asset-studio.html
https://www.npmjs.com/package/react-native-icon