For the record, OPs wrong solution is:

public class Solution{
  public static String diagonal(int[][] matrix){
    System.out.println(matrix[0][0]+" "+matrix[0][1]+" "+matrix[0][2]);
            System.out.println(matrix[1][0]+" "+matrix[1][1]+" "+matrix[1][2]);
            System.out.println(matrix[2][0]+" "+matrix[2][1]+" "+matrix[2][2]);
            System.out.println(matrix[0][0] + matrix[2][2] > matrix[0][2] + matrix[2][0] ? "Principal Diagonal win!" :
          matrix[0][0] + matrix[2][2] < matrix[0][2] + matrix[2][0] ? "Secondary Diagonal win!" : "Draw!");
    return matrix[0][0] + matrix[2][2] > matrix[0][2] + matrix[2][0] ? "Principal Diagonal win!" :
          matrix[0][0] + matrix[2][2] < matrix[0][2] + matrix[2][0] ? "Secondary Diagonal win!" : "Draw!";
  }
}

Example input it fails is:

matrix = [
  [1, 6, 1, 4, 9]
  [7, 2, 6, 2, 9]
  [1, 4, 8, 6, 5]
  [1, 6, 0, 1, 4]
  [1, 2, 5, 4, 1]
]

Your solution is shit broken, it can handle only matrices of size 3x3. It returns wrong result for matrices larger than 3x3.

Not a kata issue.