Simcha, that is simply not true. Just because your western attitude causes you to dislike something you can’t call it Assur. Up until our material minded time nobody made excuses for the Segulos mentioned in Chazal and later Sefarim, some of which are far from reminders of Mussar (think about the great lesson of half a chicken on your head — perhaps gravity).
As for this Minhag, the source is unknown. It seems to be a Mesora in Eretz Yisroel for some time. Other than the prescribed amount there is nothing mysterious about it. You add to the Simcha that is Lekovod the great Tanna and he intervenes on your behalf. The number 18 might be K’neged the 18th of Iyar, which is the day of Lag Ba’omer. The Maharal attributes significance to that day being the 18th.