Three-dimensional (3D) porous aerogels have been regarded as lightweight and effective electromagnetic (EM) absorbers, but the rational design of components with ordered macroscopic structures at low cost is still a major challenge. In this work, we constructed an ultralight biomass-derived carbon nanosheet/MXene (CNMX) composite aerogel with an ordered porous connected network via an ice template method. Two-dimensional (2D) hybrid nanosheets are parallelly arranged with constant spacing and interconnected by a tentacle bridge to form a 3D oriented porous cellular structure. Furthermore, the interlamellar spacing can be effectively tuned by regulating the content of carbon nanosheets (CNs). The optimized 3D-oriented network effectively extends EM wave transmission paths and improves impedance matching. When the thickness is 2.5 mm, CNMX3 achieves a strong reflection loss (RL) intensity of −48.5 dB and a broad effective absorption bandwidth (EAB) of 5.2 GHz under a filling ratio of only 7 wt%. Moreover, CNMX4 reaches a broad EAB of 5.8 GHz at a thickness of only 2.1 mm. Computer simulation technology (CST) analyses were also performed to reveal the microwave absorption mechanism and demonstrate the potential application of the aerogel in EM stealth technology.