When do you say two clocks are asynchronous?

can we say that clocks C2, C1 and C0 are synchronous

Yes. The whole point of a PLL is to "lock" one frequency to another (phase, actually, but it turns out to fix the frequency too). They could be asynchronous if the PLLs are malfunctioning, in which case the output from the PLL could be a free-running clock (worst case).

Two clocks are asynchronous if they do not depend on each other, for example two free-standing oscillators at the exact same frequency will still be asynchronous, since you will always have a small amount of drift and an unknown phase at startup.

I would be very cautious in stating that the frequencies are synchronous.

With ideal PLLs that had no phase jitter then you may be able to make that claim, but in reality there will be some variation in the clock edge timing. As such, if you were driving logic using multiple clocks there may be asynchronous race conditions dictated by that phase jitter.

The clocks may be "in-tune" but synchronous is an over-statement. In reality, a PLL will be in a continual state of going in and out of synchronism. Whether that is "close enough" with a specific design of PLL for your particular requirements is another matter.

To my understanding meaning of "asynchronous"/"synchronous" may vary depending on the context but in very most cases synchronous means that events happen at a fixed phase relation.

So in your case I'd say yes, the clocks are synchronous because phases are fixed (=locked < Phase Locked Loops), although they may have different frequencies and although there may be some small jitter (phase noise).