Direct (Shunt) Current Sensing

When using a shunt current sensor and feeding the differential amplifier output to the Arduino, is it required to put some kind of “isolation” mechanism?

Are you using something like this?

Are you using something like this?
INA219 High Side DC Current Sensor Breakout - 26V ±3.2A Max [STEMMA QT] : ID 904 : $9.95 : Adafruit Industries, Unique & fun DIY electronics and kits

Not exactly that, but quite similar:

What current are you measuring?
Have you seen this?

I will be measuring the current from several 12V batteries in parallel. The current will be around 50mA to 300mA. I plan to use an 1 ohm resistor as the shunt.

I have seen the hookup guide, but I'm not sure if the sensor provides enough electrical isolation between the battery and the measured output (to be fed into an Arduino analog pin).

I would use a Hall effect current sensor to ensure isolation, but those are grossly inaccurate for such low currents.

IMO you will be just fine with a direct connection.