Oh what makes you say as such?
Not me, Ms. Fraser. And I agree. LXVI was poorly suited for government, he was strong when he was alone and weak when he needed to be strong. He dismissed minister after minister following his promises to support their ideas, but the minute he encounters any resistance, instead of standing by the minister, he sends the minister packing. His lack of confidence - I've seen it suggested that it stemmed from the fact that he was unable to consummate his marriage (among other things) - meant that he generally took the conflict-avoidance route. He was depressed after the Dauphin died in 1789, and if one sets any store by Kevin Leman's theories on the birth order (and it's influence on the personality), LXVI exhibits
almost all the traits of a middle/second-born child (the conflict avoidance, the lack of confidence (due to there being an older sibling of the same sex who takes the shine), a personality opposite to the older sibling (and from what I've read, Bourgogne was very much an extroverted child (much like many other pairs of brothers, there are stories that he bullied LXVI when they were children, too, for his shyness and his timidity), the indecision, the attempts to abjure responsibilty (his hobbies of hunting and making locks when he should have been reigning and making love to his wife instead) etc). And Bourgogne dying did no help to anyone - since by then LXVI was ten already, and Leman posits that the birth-order is concretized in the first five to seven years of life.
EDIT: And yes, I know I'm basing this on pop-psychology.
But, if one looks at both Louis XV and Louis XVI (as well as Charles X), one sort of sees a pattern that Leman's theories fit very well. Second-born child thrust into the limelight after the death of the much brighter, much bolder, much stronger (whatever) older sibling, and instead of being allowed to mourn the loss like a normal child would, it gets put upon him to fill the older child's shoes immediately, so there's that amount of stress from a young age too, I guess. Charles X is an example of the traits Leman couples to the youngest child of the specific sex in the family (irresponsible, impetuous, lovable, self-centred etc),