This is the first time I see a snippet uses goto command to negate. :P That's rather novel.

if ($active != %cm.channel) return

a /return command will get the job done nicely. For your join, part, text, action and input event, use $replace and the $event identifier to save the repetitions:

#chanmon off
on *:JOIN:%cm.channel:chatmon
on *:PART:%cm.channel:chatmon
on *:INPUT:%cm.channel:chatmon
on *:TEXT:*:%cm.channel:chatmon
on *:ACTION:*:%cm.channel:chatmon
alias -l chatmon {
  if ($istok(text action,$event,32)) {
    did -ra lime_cm 12 $calc($did(lime_cm,12) + 1)
    did -ra lime_cm 13 $calc($did(lime_cm,13) + $numtok($1-,32))
    did -ra lime_cm 14 $calc($did(lime_cm,14) + $len($1-))
    if (!$istok($nick,%cm.nick,32)) {
      set %cm.nick $addtok(%cm.nick,$nick,32)
      did -ra lime_cm 17 $numtok(%cm.nick,32)
    }
  }
  elseif ($event == input) {
    if ($active != %cm.channel) return
    did -ra lime_cm 12 $calc($did(lime_cm,12) + 1)
    did -ra lime_cm 13 $calc($did(lime_cm,13) + $numtok($1-,32))
    did -ra lime_cm 14 $calc($did(lime_cm,14) + $len($1-))
  }
  else {
    var %chatmon = $replace($event,join,15,part,16)
    did -ra lime_cm %chatmon $calc($did(lime_cm,%chatmon) + 1)
    if (!$istok($nick,%cm.nick,32)) {
      set %cm.nick $addtok(%cm.nick,$nick,32)
      did -ra lime_cm 17 $numtok(%cm.nick,32)
    }
  }
}
#chanmon end

This saves you about 200 plus bytes. And for the unset command, you can do it with one /unset

unset %var1 %var2 %var3 etc...