Phased array antennas are indeed usable antennas for radar purposes and many modern radars use them. That said just having a suitable antenna is not enough for having a radar system, you also need sophisticated equipment for both transmitting powerful enough pulses and receiving the (extremely weak) reflections. The power of the reflections drops with the fourth power of the distance to the target. From a quick look at the starlink satellites I very much doubt they could effectively function as a radar system. There's just not enough solar panels to power a big radar on there.

Background: I used to be an weapons engineering officer working on future radar systems back in my Navy days.