In our system, 86.4k seconds go in a day. In the French decimal (not metric time! That was never a part of the metric system!) timekeeping, 100k of their seconds went in a day; thus 1 of our seconds is 0.864 of theirs.
Even if decimal time was part of the metric system, in the end that’d be kind of like saying metric units of length are bad because 1 centimeter is equal to 0.3937007874 inches. It’s simply a case of one set of units not matching the metric units.
If methods of measuring time hadn’t been standardised among scientific minds in 18th century Europe, unlike all other types of units (length, area, volume, etc.) then decimal time would not be unusual at all.
