I know it’s a mitzvah to build the succah, which in our home the men and boys usually do, but it’s usually the women and girls who do the decorating. Is the decorating of the succah also a mitzva?
Decorating the succah is definitely a very important part of the mitzva.
The Sages taught (Shabbat 133b) regarding the verse, “This is my G‑d and I shall worship Him with beauty” that one is to beautify oneself before Him in the performance of mitzvot. And not only must one be adorned with mitzvot, but those mitzvot themselves must be beautified, as the Sages taught (ibid), “Make for Him a beautiful succah, a beautiful lulav, a beautiful shofar, beautiful tzitzit, a beautifully written Torah scroll …in a beautiful covering”.
So not only should a succah be beautifully decorated, but all the mitzvot should be performed with regard for esthetic beauty in accordance with the above teaching on the verse, “This is my G‑d and I shall worship Him with beauty”. Nevertheless, it is especially true of the mitzva of succah which is accompanied by the taking of the four species, about which it says explicitly in the Torah (Lev. 23:40), “And you shall take for yourselves the fruit of a beautiful tree [whose fruit is beautiful].”
Therefore we are obligated to show regard for the succah and not bring any unseemly objects into it, or do anything unseemly or mundane while in it (Sh.A., O.Ch. 639:1). Rather one should decorate it as beautifully as possible where practical, with pictures and decorations, hanging fruits, wines, flowers and ornaments, elegant curtains, lovely utensils, handsome tables and chairs, attractive lighting, rugs etc.
In this regard, it is important to know that any objects designated and used to beautify and adorn the actual succah roof or walls may not be used for any other purpose during the duration of the holiday. They may not be removed from their use as decorations, and even if they fall down, they may not be put to any use. So even fruits or wines that were used as decorations may not be consumed until after the holiday. What’s more, if they fall on Shabbat or Yom Tov, they are muktza and may not be handled. However, if one affixes them on condition that they may be used during the holiday, the condition works (ibid, 638:1-2).