Polymer-derived ceramic (PDC)-SiOC is a highly promising microwave-absorbing material characterized by high temperature resistance, lightweight, high strength, and extremely low cost. The weak electromagnetic wave (EMW) attenuation capacity and poor flexibility of single precursor-derived SiOC ceramics significantly limit their further application. This study employs a simple electrospinning technique to uniformly distribute Co and TiO₂ within amorphous SiOC nanofibers. The three-dimensional porous structure formed by continuous nanofibers endows Co/TiO₂/SiOCs with high porosity, significantly reducing the thermal conductivity and enhancing the conductive loss of electromagnetic waves within the nanofiber mats. Additionally, the introduction of Co and Ti promotes nanostructuring of the fibers and introduces polarization interfaces and defects, thereby enhancing the polarization loss of the samples. With a filler content of only 5 wt%, the Co/TiO₂/SiOC sample heat-treated at 800 °C (in silicone resin) exhibits an effective absorption bandwidth (EAB) of up to 8.64 GHz (9.36–18.00 GHz) at a thickness of 3.25 mm, achieving a minimum reflection loss (RL_min) value of −66.00 dB at 17.11 GHz with a matching thickness of 2.50 mm. Moreover, the nanofiber mats also demonstrate excellent thermal insulation performance (thermal conductivity ranging < 0.041 W·m⁻¹·k⁻¹), remarkable flexibility (the resistance change rate after 1500 cycles of 180° bending test is less than 4%), and impressive resilience performance (residual strain < 12% after 500 cycles under 60% strain conditions). The successful preparation of such multi-functional nanofiber mats is promising for the application of thermal and microwave protection.