Add new column based on whether another column's value is top N

In a custom column, put following

= if List.Contains(List.FirstN(List.Sort(List.Distinct(#"Changed Type"[Score]),Order.Descending),10),[Score]) then "Y" else "N"

See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test (later on when you use the query on your dataset, you will have to change the source appropriately. If you have columns other than these, then delete Changed type step and do a Changed type for complete table from UI again)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Pc3LCcBACATQXvYcwhj/tSzpv40sRPX0UGbce+HOyFzv9TOGUCuGGDfZvWjZSztTVPdoGp6iTCsn9Z3BUiTTIahTcKemoj9ApgFQHvKJvR8=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Score = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Score", type number}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Is Top 10?", each if List.Contains(List.FirstN(List.Sort(List.Distinct(#"Changed Type"[Score]),Order.Descending),10),[Score]) then "Y" else "N", type text)
in
    #"Added Custom"