Использовать понимание списка со словарем:
a = [{"telemetry":{a:b}} for a, b in df[['foo','bar']].to_numpy()]
print (a)
[{'telemetry': {'a': 1}}, {'telemetry': {'b': 2}}, {'telemetry': {'c': 3}}]
При необходимости jsons:
import json
b = [json.dumps({"telemetry":{a:b}}) for a, b in df[['foo','bar']].to_numpy()]
print (b)
['{"telemetry": {"a": 1}}', '{"telemetry": {"b": 2}}', '{"telemetry": {"c": 3}}']
c = json.dumps([{"telemetry":{a:b}} for a, b in df[['foo','bar']].to_numpy()])
print (c)
[{"telemetry": {"a": 1}}, {"telemetry": {"b": 2}}, {"telemetry": {"c": 3}}]