Your statement is true, of course, but it only provides a partial view of the whole picture. Therefore, it is somewhat misleading.My reference to mediocre was to someone accepting returns just slightly worse than average where average is the index and the drag is a small MER and tracking errors.
To paint a complete picture (for the sake of newbie investors), you should have mentioned that disciplined couch potato investors achieve above average results in the long run. Above average when compared to all retail investors as a group. *That* is not a mediocre result.