Yes, if there are enough objects in low-earth orbit to cascade into a Kessler syndrome type scenario, then it's plausible.
However, it's reasonable to suppose that, if the world ever had enough low-orbit infrastructure to present such a scenario, governments would impose strict monitoring and intervention on all skybound objects to protect the infrastructure. That could include mandatory inspection and authorization of all objects being launched to space, and a network of satellites dedicated to monitoring objects approaching orbit, armed with some kind of interceptor to neutralize unauthorized objects -- it maybe could be done with wide, high-powered lasers, or heavy nets with propulsion on the edges to physically catch or redirect the objects.
Under those conditions, terrorists would have to do some preparation for the attack. The hard thing is to make a threat so that you can make demands, but not follow through with it if you don't absolutely have to. You have to make it clear that you're capable of triggering Kessler, without telling "how" you'll do it, and without actually doing it, so that nobody can stop you.
Here's an example of how this might be done with some misdirection... Say the terrorist organization has a few people working in a government agency related to regulating space objects. They make the false but credible claim that they've rigged several satellites with explosives. The world governments immediately send squads, and cause satellites to begin scanning one another, to check for tampering, all while negotiating with the terrorists. The truth is that the terrorists have hacked into a few satellites which have built-in propulsion. Midway through the negotiations, the government realizes there are no bombs, and calls the bluff, saying that there's no deal. As soon as this happens, and while the satellites are still being monitored carefully, the terrorists redirect several of them into colliding orbits. The government is now aware of the threat, aware of the terrorist's demands, pressed for time, and caught unprepared for the nature of the threat, because their resources are all devoted to another task. The governments pay up, and the terrorists put the satellites back on course.