Custom Editor for MonoBehavior doesn't work

[lament niche]

I've made this script (ClassTooltipDrawer.cs), placed it in the Editor folder, aaaand... Nothing happens. Debug.Log doesn't trigger, extra LabelField isn't getting added, nothing.

    [CustomEditor(typeof(MonoBehaviour))]
    public class ClassTooltipDrawer : Editor
    {
        private string tooltipText = "test!";

        private void OnEnable() // Seems like it isn't called
        {
            var attributes = target.GetType().GetCustomAttributes();
            foreach (var attr in attributes)
            {
                if (attr is TooltipAttribute tooltip)
                {
                    tooltipText = tooltip.tooltip;
                    break;
                }
            }
            Debug.Log(tooltipText); 
        }
        
        public override void OnInspectorGUI() //Seems like this is being ignored as well?
        {
            if(tooltipText.Length > 0)
                EditorGUILayout.LabelField(tooltipText);
            base.OnInspectorGUI();
        }
    }

I want to make use of the "tooltip" attribute and show it on class in Inspector (for now - as extra field, I didn't figured out how to do popup tooltips yet).

[hot patio]

the CustomEditor attribute must specify a type for which you want the inspector to draw. If that type is a base type (and not the most derived one), you need to add editorForChildClasses:true. Not sure if you can make a custom inspector targeting all MonoBehaviours though.

[lament niche]

Not sure if you can make a custom inspector targeting all MonoBehaviours though.
But can I make one for a class that derives from MB?