The fan-favorite bounty hunter Boba Fett came face to face with Bo-Katan in The Mandalorian season 2 finale, "Chapter 16: The Rescue," and needless to say, the meeting was less than friendly, but why do Mandalorians hate Jango Fett clones? Bo-Katan dismisses Boba as a pretender and a disgrace to his armor, refusing to acknowledge Jango Fett as the father of Boba and disparaging him for being a clone. It doesn’t take long before Boba and Bo-Katan’s companion, Koska Reeves attack each other, and the reasons for this antagonism can be found in Star Wars: The Clone Wars, Star Wars Rebels, and Jango Fett’s re-canonized backstory from Legends.
In Star Wars Legends, Jango Fett and his family were caught in the middle of a Mandalorian Civil War between the true Mandalorians and a murderous splinter group called the Death Watch. After the Death Watch murdered the 10-year-old Jango’s parents, he joined Jaster Mereel and became a true Mandalorian, helping to defeat the Death Watch and eventually becoming the Mandalore (Mandalorian leader). In The Mandalorian season 2, episode 6, "Chapter 14: The Tragedy," the broad strokes of this backstory returned to canon as revealed by Boba Fett upon reclaiming his armor.
Jango was also the template for the Republic’s Clone Army, whose Clone Troopers were among the greatest non-Force sensitive warriors in the galaxy. The Death Watch was neutral for most of the Clone Wars but was briefly allied with the Separatist Alliance early in the conflict before being betrayed by Count Dooku, as seen in The Clone Wars. Clone Troopers had few interactions with the Clone Wars-era Mandalorians but notably fought against Death Watch leader Pre Vizsla and his warriors (including Bo-Katan) on Draay 2, where the clones were victorious. In the final days of the war, a detachment of Clone Troopers assisted Bo-Katan’s Mandalorian resistance against the former Sith Lord Maul’s Mandalorian loyalists, where they once again defeated their Mandalorian opponents.
Unfortunately, the true purpose of the clones was to initiate the Jedi Purge and usher the fascist Galactic Empire. Though good men, the clones were compelled via brain implants to murder their Jedi friends and facilitate the coup. Though the Empire quickly replaced the clones with recruits for their Stormtroopers, the clones were unwillingly instrumental in the rise of the Empire, which soon occupied Mandalore, robbing Bo-Katan’s resistance of their victory. As seen in Star Wars Rebels, the Empire had no respect for Mandalorian culture, using superweapons that targeted their sacred beskar armor in order to keep their warriors subjugated.
Bo-Katan and her warriors, the Nite Owls, are part of the Death Watch, and therefore fought against Jango Fett’s faction of true Mandalorians during their Civil War, likely contributing to Bo-Katan’s hostility towards any of Jango’s clones. Jango was popularly believed to have been a common bounty hunter who uses Mandalorian gear, and his apparent disinterest in correcting this misconception (largely shared by Boba) may have been even more insulting to any proud Mandalorian, let alone Bo-Katan. Of course, losing the Mandalorian Civil War and failing to defeat Jango’s clones on Draay 2 may have led Bo-Katan to further resent them. None of the clones, including Boba, follow the Mandalorian creed, yet they regularly defeat Mandalorian warriors.
Although the Clone Troopers were invaluable during the Siege of Mandalore, they also paved the way for the oppression of the Empire to take over Mandalore and indirectly remove Bo-Katan’s ruling power (and Darksaber). Because of this, any Mandalorian, Death Watch or otherwise, would likely harbor mistrust at best and hatred at worse for Jango’s clones, especially considering how little most knew about Order 66 and the clones’ brainwashing. If not for the clones, Bo-Katan wouldn't still be fighting to win back the Darksaber (and thus Mandalore) in The Mandalorian.