Can we add tooltip on cells in Excel sheet in .NET code with spire.xls?

You can add comments to Excel cells. Checking the following code example:

using Spire.Xls;

namespace Comment
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a Workbook object
            Workbook workbook = new Workbook();

            //Load the sample workbook
            workbook.LoadFromFile("Sample.xlsx");

            //Get the first worksheet
            Worksheet sheet = workbook.Worksheets[0];

            //Add regular comment to specific cell range C6
            CellRange range = sheet.Range["C6"];
            ExcelComment comment = range.AddComment();
            comment.Text = "Regular comment";                      
            
            //Save the Excel workbook.
            workbook.SaveToFile("Addcomment.xlsx", ExcelVersion.Version2016);
        }
    }
}

https://www.e-iceblue.com/Tutorials/Spire.XLS/Spire.XLS-Program-Guide/Add-Comments-in-Excel-and-Set-Comments-Format-with-C-VB.NET.html