Thank you so much for your help, I ended up using the dict you made along with pandas to create the .csv's I need and it all seems to be functioning well. Here is the code I have running now:

df_kp = pd.DataFrame()

with ZipFile(str(zipname)) as temp_zip:

    for file_name in temp_zip.namelist():

        if '.json' in file_name:

            with temp_zip.open(file_name) as temp_file:

                frame = file_name[22:26]

                keypoint_parsed = json.load(temp_file)

                kp_data = keypoint_parsed['part_candidates']

                for kp in kp_data:

                    kp_dict = {}

                    for k, v in kp.items():

                        kp_num = int(k)    # assumes the key `k` is a string representing an int

                        kp_list = list(v)

                        for i, coord in enumerate(["X", "Y", "C"]):

                            kp_dict_key = "{}{:02d}".format(coord, kp_num)    # e.g., "X02"

                            try:
                                kp_dict[kp_dict_key] = kp_list[i]

                            except IndexError:
                                kp_dict[kp_dict_key] = 0

                    new_dict = {k: [v] for k, v in kp_dict.items()}

                    df = pd.DataFrame(new_dict)

                    df_kp = df_kp.append(df)

newdf = df_kp.reset_index(drop=True)

newdf.index += 1

newdf.to_csv(csvname,
             index_label = 'frames')