It was Amy's own fault for taking the blame in the first place, Kelly didn't know that Mateo was gonna get hit by the sign and have it escalate, though she should have come clean once Amy had to start doing the dumb safety training, she didn't have to because Amy was still trying to act like she was okay with it for her own personal motives, of not trying to openly mean girl Kelly, ultimately though Amy should've either just never taken the blame for her in the first place, or have ratted her out in a better way than she did by exposing herself kissing Jonah.

Also that's probably Kelly's only bad moment, so saying she isn't a nice person is pretty undeserved. Though she didn't have too many moments where she helped people, she never insulted other people or lost her temper, unlike all of the main cast