MeshInformation.cs

using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(MeshFilter))]
public class MeshInformation : Editor
{
    public override void OnInspectorGUI()
    {
        var filter = target as MeshFilter;
        string message = "Triangles: " + filter.sharedMesh.triangles.Length / 3 + "  Vertices: " + filter.sharedMesh.vertices.Length;
        EditorGUILayout.LabelField(message);
    }
}

[CustomEditor(typeof(SkinnedMeshRenderer))]
public class SkinnedMeshInformation : Editor
{
    public override void OnInspectorGUI()
    {
        var skin = target as SkinnedMeshRenderer;
        string message = "Triangles: " + skin.sharedMesh.triangles.Length / 3 + "  Vertices: " + skin.sharedMesh.vertices.Length;
        EditorGUILayout.LabelField(message);
    }
}