First you need one png file as your launcher icon.
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.
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.
open-app
2.use Image Asset Studio
open-image-asset
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.
ic_launcher
If your app supports versions no higher than Android 7.1, create a legacy launcher icon only.
ic_launcher-1

Check the result
react-native run-android

lc

Refer

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

Create App Icons for React Native Android App