Well, your kinda right. In 98se it shows the keystroke (if listed on the keyboard) and if its not, then shows ascii. If you really want a one for mirc though, I posted one about a coule weeks back that ppl seemed to enjoy.
Code:

/asciitable {
  /window @ascii
  /aline $color(info) @ascii Ascii: $chr(9) Value:
  /aline $color(info) @ascii $str(-, 25)
  /set -u0 %ascii 0
  while (%ascii <= 255) {
    /unset %info
    if (%ascii == 0) /set -u0 %info $+($,NULL)
    else if (%ascii == 2) /set -u0 %info BOLD (Ctrl+B)
    else if (%ascii == 3) /set -u0 %info 12C4OL8OR (Ctrl+K)
    else if (%ascii == 13) /set -u0 %info Enter/Return
    else if (%ascii == 15) /set -u0 %info PLAIN (Ctrl+O)
    else if (%ascii == 22) /set -u0 %info REVERSE (Ctrl+R)
    else if (%ascii == 27) /set -u0 %info Escape
    else if (%ascii == 31) /set -u0 %info UNDERLINE (Ctrl+U)
    else if (%ascii == 32) /set -u0 %info (Space)
    else if (%ascii == 160) /set -u0 %info (Hard Space)
    /aline $color(info) @ascii %ascii $chr(9) $+($chr(%ascii),) $chr(9) %info
    /inc -u0 %ascii
  }
  /aline $color(info) @ascii $str(-, 25)
  /aline $color(info) @ascii Ascii: $chr(9) Value:  
}

-KingTomato