Adds context menu items to assets: Copy/Paste import settings & Copy GUID. Put this script in any Editor/ folder in your project and recompile!

AssetCopyUtils.cs

// written by https://github.com/FreyaHolmer so use at your own risk c:

using System.Collections.Generic;
using UnityEditor;
using UnityEngine;

/// <summary>Utility functions to copy GUIDs, as well as Copy/Paste import settings</summary>
public static class AssetCopyUtils {

	const int CTX_MENU_LOCATION = 70;

	[MenuItem( "Assets/Copy GUID", false, priority = CTX_MENU_LOCATION, secondaryPriority = 0 )]
	public static void CopyGUID() => GUIUtility.systemCopyBuffer = string.Join( ", ", Selection.assetGUIDs );

	[MenuItem( "Assets/Copy GUID", true )]
	public static bool CopyGUIDValidate() => Selection.assetGUIDs.Length > 0;

	static AssetImporter sourceImporter; // todo: doesn't survive assembly reload but I also don't care that much

	[MenuItem( "Assets/Copy Import Settings", false, priority = CTX_MENU_LOCATION, secondaryPriority = 1 )]
	public static void CopyImportSettings() {
		string path = AssetDatabase.GetAssetPath( Selection.activeObject );
		sourceImporter = AssetImporter.GetAtPath( path );
	}

	[MenuItem( "Assets/Copy Import Settings", true )]
	public static bool CopyImportSettingsValidate() {
		if( Selection.count != 1 )
			return false; // too many things selected
		if( AssetDatabase.Contains( Selection.activeObject ) == false )
			return false; // not an asset
		return true;
	}

	[MenuItem( "Assets/Paste Import Settings", false, priority = CTX_MENU_LOCATION, secondaryPriority = 2 )]
	public static void PasteImportSettings() {
		AssetDatabase.DisallowAutoRefresh();
		try {
			HashSet<string> paths = AllValidPasteTargetAssetPaths();
			foreach( string assetPath in paths ) {
				AssetImporter targetImporter = AssetImporter.GetAtPath( assetPath );
				Undo.RecordObject( targetImporter, "Paste import settings" );
				EditorUtility.SetDirty( targetImporter );
				EditorUtility.CopySerialized( sourceImporter, targetImporter );
				targetImporter.SaveAndReimport();
			}
			Debug.Log( $"Pasted {sourceImporter.GetType().Name} settings into {paths.Count} {( paths.Count == 1 ? "asset" : "assets" )}:\n{string.Join( "\n", paths )}" );
		} finally {
			AssetDatabase.AllowAutoRefresh();
		}
		AssetDatabase.Refresh();
	}

	static HashSet<string> AllValidPasteTargetAssetPaths() {
		HashSet<string> assetPaths = new();

		// todo: I couldn't get folders to work, bc for some reason editing assets that are not selected, makes for an Undo stack horror
		// find folders and assets in selection
		// List<string> folderPaths = new();
		foreach( Object selObj in Selection.objects ) {
			if( AssetDatabase.Contains( selObj ) == false )
				continue; // not an asset
			string path = AssetDatabase.GetAssetPath( selObj );
			if( IsFolder( selObj ) ) {
				// folderPaths.Add( path ); // to find all guids of assets in the folder
			} else if( ObjectIsValidPasteTarget( path ) ) {
				assetPaths.Add( path );
			} else {
				// something was explicitly selected, but doesn't match the copied type, pls warn
				Debug.LogWarning( $"Can't paste {sourceImporter.GetType().Name} settings onto {selObj.GetType().Name} \"{selObj.name}\" at {path}" );
			}
		}

		// then, also add all matching assets in the selected folders, if any
		// if( folderPaths.Count > 0 ) { // deeply crucial if statement here do NOT remove lol
		// 	foreach( string assetPath in AssetDatabase.FindAssets( "", folderPaths.ToArray() ).Select( AssetDatabase.GUIDToAssetPath ) ) {
		// 		if( ObjectIsValidPasteTarget( assetPath ) )
		// 			assetPaths.Add( assetPath ); // add non-folder assets found that have the same import type
		// 	}
		// }

		return assetPaths;
	}

	static bool IsFolder( Object obj ) => AssetDatabase.Contains( obj ) && IsFolder( AssetDatabase.GetAssetPath( obj ) );
	static bool IsFolder( string path ) => AssetDatabase.IsValidFolder( path );

	[MenuItem( "Assets/Paste Import Settings", true )]
	public static bool PasteImportSettingsValidate() {
		if( Selection.count == 0 )
			return false;
		if( sourceImporter == null )
			return false; // copied data got assembly reloaded
		return true;
	}

	static bool ObjectIsValidPasteTarget( string path ) => IsFolder( path ) == false && AssetDatabase.GetImporterType( path ) == sourceImporter.GetType();

}