Loki is more fleshed-out than TDK Joker.
Joker:
- Appears out of nowhere, barely any backstory or relation.
- No redeeming factor.
- No logical explanation to how he can motivate untrained thugs into completing logistically impossible tasks in parallel. When audience goes into each set piece, the elaborate trap has already been magically set up, all behind the scenes
Loki:
- Carefully established, has fleshed out backstory.
- Sympathetic flawed character, motivations are understandable.
- You can see him trying to execute his plans, including recruiting ex-SHIELD people, bargaining with Thanos, failing and regrouping.