Yes but Kim had not been gone for that long and the corpse looked very like Kim and given it was in Kims car Zoe would have no reason to think it was not Kim and would expect the body to look slightly different to usual and Frank was for reasons only known to him at that time sure it was Kim so would have no doubts, but as we also know it fitted the storyline Dental records are not always accessible, and fingerprints unlikely to be on records without a criminal history. You're right, but I imagine part of the the development of the character was to have him be hot to a mid 20s woman, lose his virginity to her and have an affair, and Chris Smith didn't feature in these plans as he was too young looking. In addition to the disfiguring bloating, Frank and Zoe would've been expecting the body to be Kim's, so simply saw what they expected to see. Kelly is young looking and the actress is the same age as her character, 16, yet that didn’t stop them letting her character have an affair with a teacher.

janeetherton37 - Pinterest

She would have been discoulered and bloated etc but Kim chose someone who looked similar to her and Frank well given what we know would identify it as Kim as he did not expect it to be anyone else. Given they were investigating him for Kim's disappearance Frank falsely identifying a corpse as Kim would be regarded as extremely unlikely.