The people could have shown remorse by weeping, shaving their heads, and wearing mourning clothes. If they were focused on God, this would have been their reaction. Instead, they recklessly tried to take what they could, while they could. This shows their fatalistic, mocking attitude. God did not call for mere outward signs of mourning (Joel 2:13a), but true repentance is also expressed by outward signs.
Eating and drinking in itself is not wrong. It is the careless self-indulgence that is displeasing to God. Joy and gladness are good, unless it is not in the Lord.
12 In that day the Lord GOD of hosts called for weeping and mourning, for baldness and wearing sackcloth;