The fact that the Mandalorians hate the Jedi is a widely known fact in the world of Star Wars. The reason for this hatred probably has something to do with a generation-spanning series of conflicts known as the Mandalorian-Jedi War.
From Star Wars: The Clone Wars to The Mandalorian, this war has been alluded to as a remarkably dark time in Mandalorian history. Mandalorians are not ones to forgive easily, and they certainly are not the type of people to let grudges fade, even well into the time after The Empire.
The Mandalorian-Jedi War
How long the war between the Jedi and the Mandalorians lasted isn't clear, but considering that in The Clone Wars Pre Vizsla claimed that his family fought the Jedi “for generations,” it likely lasted a century or two. It’s also likely that the war was more probably a series of conflicts spanning decades, against both the Jedi Order and the Old Republic. This war allowed for the Mandalorians to advance as warriors, likely due to the particular challenges of fighting Jedi on the regular. The reason the conflict began is currently unknown in canon, though in Legends the conflict is referred to as the Mandalorian Wars and began as a result of Mandalorian expansion and conquest clashing with the Old Republic.
The war culminated in a battle on Mandalore called the Mandalorian Cataclysm. In Clone Wars, Mandalore's inhabitants lived in futuristic bubble cities, and in The Mandalorian, Din Djarin mentions that Mandalore is a cursed planet. The surface of Mandalore became uninhabitable due to this final battle against the Jedi, reducing the planet to a desert, bearing the physical scar of the Mandalorian’s loss to the Jedi. In Star Wars: Rebels, Sabine Wren, a Mandalorian, reminisced about a planet once covered in grass before admitting that she'd never actually seen it, as Mandalore had become a wasteland long before she was born.
The Grudge Against the Jedi
The Mandalorian-Jedi War ended long before the events of Clone Wars, but the grudges from that era carried over. The fact that the Darksaber, a weapon made by the first Mandalorian Jedi and stolen during this conflict, is a symbol of power among the Mandalorians is a reflection of this. Pre Vizsla, who wielded the Darksaber for a period of time during Clone Wars, was proud of his family’s involvement in the war. In The Mandalorian, the Armorer tells Din Djarin about the Jedi, but describes them only in terms of their role in the Mandalorian-Jedi war, causing Din to refer to them as “enemy sorcerers.”
However, the Armorer’s view of the Jedi might be a bit antiquated, given her status as a Children of the Watch member. Considering this sect of Mandalorians split off from Mandalorian society, according to Bo-Katan Kryze, the Armorer’s perspective on the Jedi is probably not one shared by all Mandalorians. Bo-Katan herself was willing to work with the Jedi during the seventh season of The Clone Wars, aligning herself with Ahsoka Tano, who in turn asked the Jedi Order for aid in reclaiming Mandalore.
Mandalorians Working with Jedi
In The Mandalorian Season 2, Episode 5, Ahsoka Tano jokes with Din Djarin about the irony of a Jedi and a Mandalorian working together, implying that, despite her having worked with Mandalorians in the past, such alliances were still fairly uncommon. Even in Star Wars: Rebels, whose Ghost rebel crew included Kanan Jarrus, a Jedi Knight, his Padawan Ezra Bridger and Sabine Wren, a Mandalorian, biases between Jedi and Mandalorians, though faded, still came up on occasion. Sabine usually worked with her Jedi teammates with little issue -- in spite of also being from House Vizsla -- but when Kanan Jarrus had to instruct her on wielding the Darksaber, much of his frustration with the process was blamed on his biases against her Mandalorian background.
Whatever the time period, the main thing to note is that, despite their shared sour history, Mandalorians and Jedi will still put aside their grudges in the face of a common enemy. AS with Maul in Clone Wars and the Galactic Empire in Rebels and The Mandalorian, the Mandalorians are a people adept and adapting to survive. However, the Mandalorian-Jedi War is what forged the Mandalorians into the warriors they are, so it’s unlikely they will forget it even if they collectively decide to forgive it.