While it has been quite some time since Jigen was defeated in the Boruto anime, fans must remember him for being overpowered, even against the likes of the greatest shinobi like Naruto and Sasuke. So, why was Jigen so strong in Boruto: Naruto Next Generations?
Jigen was so strong in the Boruto anime because he was Isshiki Otsutsuki's host. Additionally, he had amped up his powers with the Ten-Tails chakra before fighting Naruto and Sasuke. This is why he fared better against them than fused Momoshiki Otsutsuki.
Jigen's status as Isshiki's host, explained

As revealed in the anime, unlike the other Otsutsuki hosts like Kawaki and Boruto, Jigen was an imperfect host to Isshiki Otsutsuki due to his special circumstances. When Isshiki Ostutsuki first arrived on Earth with Kaguya Otsutsuki, Kaguya left him for dead. Amidst this, Isshiki shrank himself and entered a monk's ear to survive. This monk was none other than Jigen.
In time, Isshiki Otsutsuki gained full control over Jigen's body and embedded his karma in him. Unfortunately, as Jigen wasn't powerful enough to sustain Isshiki's chakra, the Otsutsuki chose not to resurrect himself in Jigen's body.
So, while using Jigen's body, Isshiki looked for a more suitable host body. For this, he created the group called Kara and later found his "perfect vessel" in Kawaki. Effectively, for Isshiki, Jigen's body was just a means to find a better vessel and not someone he wanted to resurrect himself in.
Jigen's other powers and abilities in Boruto

Besides the power Jigen drew from Isshiki Otsutsuki through Karma, the monk had Amado modify his body with Scientific Ninja tools and Shibai Otsutsuki's DNA. This was why Jigen lived for over 1000 years and became the strongest Kara member, surpassing all Inners.
Speaking of Shibai Otsutsuki's DNA, Jigen had derived several Shinjutsu from Isshiki Otsutsuki. He could produce black rods capable of draining chakra from his victim. Additionally, he could shrink himself and other matter to microscopic levels and revert them to their original sizes instantly.

He could utilize these abilities by piercing his opponent with microscopic black rods. While his opponent was left unaware, Jigen could revert the black rods into their original sizes, inflicting catastrophic injuries almost instantly.
Similarly, Jigen could instantly turn himself into a microscopic size and disappear from his opponent's sight to manoeuvre around the battlefield and appear elsewhere. The anime never showed any downside to this technique, but showed Sasuke detecting Jigen's moves using his Sharingan.

Jigen could also manifest various objects stored within his pocket dimension and bring them out with the most minute gestures. He could enlarge or reduce the sizes of these objects as he pleased to fight against his enemies.
Related links:
- Mitsuki’s biggest moment in Boruto may be coming soon (& Jura is the key)
- Boruto's defeat at Jura's hands is all but sealed (& his friends are why)
- Jura's target in Two Blue Vortex chapter 22 may not be Boruto (& it makes perfect sense)