Detection of cracking in cementitious composites through their own electrical feedback is of great interest, as these materials can act as both structural elements and sensing components, providing early warnings that enhance durability and integrity. However, the electrical signals generated by self-sensing cementitious materials are often weak, making them difficult to distinguish from signals caused by other environmental factors. These signals are also influenced by matrix properties, particularly porosity, which has rarely been investigated. This study investigates the crack-sensing performance of concrete composites incorporating single carbon nanotube (CNT)-coated polyethylene (PE) fibers within both high-strength (HS) and normal-strength (NS) cementitious matrices. Electrical characteristics, porosity, and crack-detection behavior were evaluated to determine how matrix type affects self-sensing capability. Mercury intrusion porosimetry (MIP) showed that the porosity of the NS matrix is nearly twice that of the HS matrix, creating a more open fiber–matrix interface. As a result, NS specimens exhibited lower resistivity and higher conductivity than HS specimens. During crack development, the relative resistance of HS specimens increased by up to 4000 % for crack widths exceeding 200 μm, far lower than the 100,000 % change observed in NS specimens. This heightened sensitivity is attributed to greater fiber mobility and CNT detachment in the more porous NS matrix, which disrupts electrical pathways. In contrast, CNT displacement is restricted within the denser HS matrix, resulting in a more stable but less sensitive response. These findings highlight the critical role of matrix selection in optimizing the self-sensing performance of concrete composites for structural health monitoring.