AncientPower
The Chosen One
She broke through the instinctive Force barrier of a Meetra who was still getting to her feet and in a compromised position. Hardly the same thing as mounting a genuine defense. Force barriers, as LeGenD has pointed out before, are not designed to counter Force lightning. It takes Tutaminis to block/absorb/reflect Sith lightning, and even the most powerful Jedi Masters have difficulty countering such attacks with said technique.
Given all of that, it's actually very impressive for Meetra's instinctive barriers to be capable of absorbing the worst of Nyriss' lightning.
We know Dromund Kaas is that powerful, because the most powerful Jedi of all time (FOTJ Luke) went there thousands of years after the planet was at its most potent, and described the energy there as 'psychic sewage' which slowed both his and Jaina Solo's reflexes so much that random Sith Sabers, who were amplified by that power, became a decent fight.
Not to mention corrupting Kyle Katarn to the dark side and blinding Mara Jade from the light side of the Force.
We also have the fact that Lord Scourge, at the beginning of the novel, states that he could empower himself through the dark side energy 'pulsating' from the Dark Citadel to keep him going for hours. Despite being on the very outskirts of the city.
Furthermore, we know for a fact that the Emperor transformed Dromund Kaas with a ritual which saw the atmosphere permanently tainted with dark side miasma, something Meetra reacts to immediately upon entering orbit of the planet.
Just so you know, dark side miasma is the same storm-like taint which is seen within the tombs of ancient Sith Lords such as Ludo Kressh and Freedon Nadd, which could cause hallucinations in the Exile's mind and corrupt Jedi such as Visas Marr, Brianna, and Mical on contact.
Dromund Kaas is essentially engineered by Vitiate to be extremely potent with the dark side of the Force.