From the very start they don’t like her. She’s ruthless and cruel, and manages to upset everyone in the castle, except Filch who seems to delight in her draconian methods of punishing students. She’s unable to do more damage because of Dumbledore’s presence in the castle, so the Marauders are content pulling the occasional prank on her where they can.
It’s not until she goes on a seemingly endless prejudice-fuelled anti-werewolf tirade, after which Remus barely speaks for a fortnight, that dislike turns into outright loathing. After that, at least in the minds of James, Sirius and Peter, it’s open season on Umbridge. Umbridge finds herself pranked, underminded, and hindered at every turn. The simplest tasks take hours, yet still manage to go wrong and her fellow teachers turn a blind eye to any trouble she’s having.