"Instead of going after the people that did it, the judge went after her and went after her department, and I guess she choked. I don't know what the hell happened," Trump told reporters on Monday.
Pirro's office said in a court filing on Friday that the damage resulted from shoddy construction, not vandalism, and moved to drop charges against former Olympian David Hearn, who was accused of deliberately damaging the newly renovated pool ahead of the nation's 250th birthday celebration in July.
The renovated pool had quickly developed blooms of algae that tinted the water green, while pieces of the blue lining had peeled off.
Trump, facing criticism over the $US14.7 million ($A21 million) renovation, had repeatedly blamed the problems on vandals he alleged had intentionally sabotaged the project.
The president claimed surveillance footage showed people cutting newly installed waterproofing material around the pool and argued that Pirro, a close political ally, should have continued pursuing the case.
"Pirro made a mistake. It was vandalism," Trump said, adding he was "really disappointed" in Pirro and that "she folded like an umbrella".
Pirro, a former Fox News host and longtime Trump ally, is the top federal prosecutor in Washington.
Trump did not say whether he had spoken directly with her before publicly rebuking her and did not answer when asked if he was reconsidering her role.
Pirro's office and the White House did not immediately respond to a request seeking comment.
The reflecting pool renovation is among several projects Trump has pursued across Washington. He has demolished the White House East Wing to make way for a $US400 million ($A572 million) ballroom and proposed a large arch between the Lincoln Memorial and Arlington National Cemetery.
