datatextformatstring is not working

Do you mean like another culture than en-US?

if so you can use an IFormatProvider, such as CultureInfo, in such case you could simply change the cultureinfo of the thread you're operating on.
For instance say da-DK!
This culture have , as digit seperator and ; as list seperator and . as thousands seperator instead of the en-US that use . for digits and , for lists.

in this case you'd be able to use

DataTable dtscore = (DataTable)Session["GrdSearch"];
DataView dvscore = dtscore.DefaultView;
DataTable distinctscore = dvscore.ToTable(true, "Score");
distinctscore.Locale = new CultureInfo("da-DK");
ddlscore.DataSource = distinctscore;
ddlscore.DataTextField = "Score";
ddlscore.DataTextFormatString = "{0:0.0%}";
ddlscore.DataBind();

I'm updating solution with a somewhat more explicit implementation to examplify and also updating where the Locale goes as i'd put it on the control not the table by mistake B-]. My page has a single dropdown control with the name DropDown.

using System;
using System.Data;
using System.Globalization;

public partial class DataTableBinding : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            DataTable dataTable = GetTable();            
            DataView dataView = dataTable.DefaultView;
            DataTable dataTableDistinct = dataView.ToTable(true, "Score");
            dataTableDistinct.Locale = new CultureInfo("da-DK");
            DropDown.DataSource = dataTableDistinct;
            DropDown.DataTextField = "Score";
            DropDown.DataTextFormatString =  "{0:0.0%}";
            DropDown.DataBind();
            DropDown.Items.Insert(0, "ALL");
        }
    }

    private static DataTable GetTable()
    {
        DataTable dataTable = new DataTable();
        dataTable.Columns.Add("Score", typeof(decimal));
        DataRow row = dataTable.NewRow();
        row["Score"] = 0.98;
        dataTable.Rows.Add(row);

        row = dataTable.NewRow();
        row["Score"] = 0.97;
        dataTable.Rows.Add(row);

        row = dataTable.NewRow();
        row["Score"] = 1.00;
        dataTable.Rows.Add(row);

        row = dataTable.NewRow();
        row["Score"] = 0.85478;
        dataTable.Rows.Add(row);

        return dataTable;
    }