You can even shrink the alias more by using regsubex for the reverse part of the code:

alias -l mimic {
  if ($left($1,1) == !) { halt }
  if ($nick == %mnick) && (# == %chnl) {
    if (%mimic) {
      $iif($event = text,msg,$iif($event = notice,notice,$iif($event = action,describe))) # $1-
    }
    elseif (%rmimic) {
      $iif($event = text,msg,$iif($event = notice,notice,$iif($event = action,describe))) # $regsubex($1-,/(.)/g,$mid(\A,-\n,1)))
    }
  }
}

You don't have to repeate the %mnick and %chnl variables. Just make an if condition will get both routines covered.