Putting smiley faces on critical comments is likely to encourage putting smiley faces on anything that may be perceived as negative, which in turn will lead people to put smiley faces on actual hostility. Putting a smiley face on hostility just turns it into slightly more passive aggressive hostility (how dare you react to this as if it's hostile, see, I put a smiley face on) and should be discouraged.

I also worry that if we start putting smiley faces on critical comments, we'll get to the point where it's expected and someone whose comments are perceived as hostile will be told "it's your own fault--you should have put a smiley face on".