Why not?

123456
Across
  1. 1. Common and generic insult that doesn't use any words.
  2. 3. "ur"
  3. 5. State of water below 0°C.
  4. 6. n^3
Down
  1. 1. Knock knock jokes do not fall into this category.
  2. 2. This happens to bad people with their ice cream.
  3. 4. 21st letter of the alphabet.