You could disable DiagramView.ShowToolTips to stop using the standard tooltips, and display a separate text control over the view as a tooltip when the mouse hovers over a node. Alternatively, you might create a ShapeNode displaying the tooltip text, which will also let you draw an arrow from the tooltip to the node as in the Excel screenshot:

private ShapeNode ttipNode;

private void diagram_NodePointed(object sender, NodeEventArgs e)
{
	if (ttipNode != null)
		diagram.Nodes.Remove(ttipNode);

	RectangleF r = e.Node.Bounds;
	r.X += r.Width + 5;
	r.Y -= 10;
	ttipNode = diagram.Factory.CreateShapeNode(r);
	ttipNode.Text = e.Node.ToolTip;

	DiagramLink link = diagram.Factory.CreateDiagramLink(ttipNode, e.Node);
	link.AutoRoute = false;
	link.Shape = LinkShape.Polyline;
} 

With whatever approach you take, you will also have to hide the tooltip object after a timeout or when the mouse moves, from some timer or mouse event handlers.

I hope that helps,
Stoyan