How do you role play a Thief in a way that is not unfun? Do that. Except it's even easier, because Kender aren't thieves.

That was the entire point. Halfling Thieves were a core part of D&D, but fat sit-at-home Hobbits and intentionally ripping off your team mates maliciously weren't fun. So they reskinned the combo into something that is fun instead.