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 is a good choice because it is in mass production (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 required,. Maybe you need more DDR memory? Or, you need 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 use it. For a proof of concept, the RPi is a good choice as well. Some 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.
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