Well, for one... Louis the XVI was considerd ineffectual because he was usually depicted as rather a weak sort of king. Not really able to wield the strong centralized state in which his ancestors had crafted.
*And he was also seen as an honest man, who had good intensions, but who was most likely unfit for the strong task of reforming the monarchy; He basically wasnt the best man for the job...
~Therefore, He was ineffectual, just like the question states...
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~Hope this helped! Anymore questions, please feel free to ask. :)