Who was the weakest player to ever beat the mature Bobby Fischer in a tournament or match game? I understand that it's a bit hard to define the "mature" Fischer indeed, since he was incredibly strong even as a teenager, but I mean roughly, let's say, from the early 1960s on, e.g. not counting his very early years.
Cesar Munoz from Ecuador is the last untitled player to beat Fischer, in 1961. Kovacevic is the last non-grandmaster to do that in 1970, but obviously he was already very strong. Janosevic in 1967 was a grandmaster but not the most famous of them. And Spassky in 1992 was not that strong anymore...
From 1960 onwards, Bobby Fischer lost 100 chess games that are registered here.
You can browse this list to pick whom you consider "the weakest" winner. About a half of the games were played in simultaneous displays so they don't qualify as tournament or match games. The answer might also depends whether you include blitz and rapid tournaments.
Fischer lost a couple of games to non-grandmasters in 1960 (IM Bernardo Wexler) and 1961 (Cesar Munoz, The unknown Ecuadorian caused a sensation in the preliminaries of the Leipzig Olympiad when he beat Bobby Fischer.), but if we limit ourselves to 1963 onwards, my suggested answer would be Dragoljub Janosevic in Skopje, 1967. It seems any other person who beat Fischer after 1963 was a world-class grandmaster (technically Kovacevic was still an IM in 1970 when he beat Bobby Fischer in Bled, but he was already of GM-strength).
Since Janosevic was a (less famous) grandmaster not far from his peak in 1967 who also won games against Tal, Petrosian, Larsen, Bronstein and an aging Botvinnik, there is also an argument to be made that the weakest person to beat the mature Fischer would actually be former world champion Boris Spassky in Belgrade, 1992, when his level of play had already gone down.