In Persuasion, are people who act foolishly in the novel always to blame for their actions?

That depends on the character's rationale behind their
action.

One of the most foolish characters is Sir Walter, and
Elizabeth would be his counterpart.  In the case of Sir Walter, his foolish behavior is a result
of years and years living among the aristocrats without having to worry about money. Then, as his
finances come to an end you would think he would have more common sense and accept his friend's
advice. But no, his foolishness prevents him from doing
so.

Elizabeth and Mary are also foolish in their behaviors but in
their case is a result of their upbringing and the tendency to mimic their father. Therefore,
although they are adults and they have the choice to monitor their own behavior like Ann did,
they may lack the attributes which Anne enjoys such as
independence.

So it is partly their fault and partly
circumstances.