The raspberry pi is quite big, so no. The raspberry zero is much smaller but is also specific. 1GHz, 512MB DDR (LPDDR2, we are at LPDDR4 for low power), SD slot, mini HDMI, micro USB and GPIO’s. For DIY projects, it could be a good choice because it is mass produced (cheap and well supported). However, specific IoT applications have their own requirements. If battery operated, low power, if not, maybe higher power consumption allowed. If the application is not CPU intensive, 1GHz clock speed is overkill. Most applications do not require the GPIO’s or just a few. Might not require the micro USB or the mini HDMI. Or the DDR memory is not needed, or more is needed or a different DDR tech (LPDDR4 for even more low power maybe). And the communication protocol for the IoT application could be proprietary, could be wifi, BT, zigbee, … . Hobbyist and maybe proof of concept could be done on a raspberry (zero) but most applications, will require a custom developed board (PCB) with the application specific processor, memory and interfaces.
Hardware and software advisor for tech startups. ASIC, FPGA, RPi, Arduino, AI, robots, drones, blockchain, Machine learning, vision processing, IoT and 3D printers are my fields of expertise.