My question is: Can a Bard make a good thief?
Short answer: Yes.
Long answer:
Any class can be a good "Thief" in the sense that it's easy for anyone to get good Dexterity scores and Proficiency in Sleight of Hand (And Stealth if you care for it and don't just use Invisibility) so they can open locks, disarm traps and pickpocket to their hearts content. Hell, I've had parties where my character with 8 Dexterity was good enough to open every lock and disarm every trap in the game thanks to items to buff them up.
Bard is able to get an extra bonus, much like a Rogue, with their Expertise allowing them to gain additional proficiency bonuses for a selected skill (Such as Sleight of Hand). Though, this isn't particularly necessary most of the time.
In terms of "Assassination", Bards do have some capacity for that. Between things like using Thunderwave (Or College of Swords Blade Flourish) to knock enemies into insta-death pits or using Hold Person/Monster to stun a target and make every melee range attack guaranteed critical they can quickly remove a target.