Rotational molding is a shear-free technology to produce hollow plastic parts. Rotomolded poly(lactic acid) (PLA) and its biocomposites remain in a largely amorphous state, which limits their stiffness, toughness, and thermal resistance. Post-processing thermal annealing develops crystallinity without additives, but its combined effect with natural fibers has rarely been quantified. This work evaluates annealing at 100 °C for 1 h on rotomolded PLA biocomposites reinforced with 10, 20, and 30 wt.% of agave, coir, or pine fibers. Crystallinity (DSC, XRD), density and porosity, morphology (SEM), water absorption, mechanical properties (tensile, flexural, Charpy impact, Shore D hardness), and 28-day disintegration under lab-scale composting conditions were measured for treated and untreated samples. Annealing raised the matrix crystallinity from below 21% to 41–56% and produced predominantly α crystals with a nearly constant average size of about 20 nm. The treatment improved the matrix-dominated properties for every formulation: for neat PLA, Charpy impact strength increased by 120% (28.1 to 61.7 J/m), flexural strength by 53% (61.1 to 93.4 MPa), and flexural modulus from 3264 to 4511 MPa. Tensile strength and modulus, in contrast, remained unchanged or decreased. Porosity, set by fiber content, was unaffected by annealing; at 30 wt.%, the matrix barrier nonetheless reversed, and annealed samples absorbed more water and disintegrated faster than untreated ones. Fiber content sets the balance between matrix crystallinity and the interfacial damage caused by crystallization-induced contraction.