Could be several things:
- the dumped rom is correct but crc32 is not found in n64.txt -> google for the crc32 to see if it is listed as a valid dump
- the dumped rom is incorrect because the cartridge is not inserted dead center into the slot -> use the 3d printed sleeve for the n64 slot to make sure carts are always aligned
- the dumped rom is incorrect because you accidentally used 5V instead of 3.3V or forgot to desolder the two components from the Arduino Mega allowing it to run at 3.3V
- dump is incorrect because there is a bad contact in the 3.3V line -> use a multimeter and measure the resistance between the two points marked by red circles while the cart reader is switched to ON and 3.3V but not connected to a power source, it must be around 0.5 Ohm but not over 1 Ohm, if it is over 1 Ohm you need to switch the switches on and off a few times to remove corrosion or even resolder the connections:

-the dumped rom is incorrect because the Arduino Mega you're using can't handle 3.3V very well -> you can reflash the Arduino Mega's fuses to disable the brown out fuse.
- the dumped rom is incorrect because there still might be an error in my dumping code that only affects certain cartridges