Royal Navy and British army Harriers (effectively equivalent to the AV-8A, the predecessor of the AV-8B) trounced Argentine Mirage fighters, French built aircraft arguably as good then as anything the Chinese have now. If the Chinese improved their training and had modern electronics and missiles equivalent to what we have, the fight would be more even, but I would not count the Harrier out - all other things being equal.
Again, the one advantage the Harrier has over other jet aircraft is that it can be hidden out in the boondocks, dispersed, so as to avoid being wiped out on the airfield, as will surely happen to most of Taiwan's air force if China decided to do so. Thirty Harriers may not be able to do much in the face of overwhelming numbers, but it is significantly better than nothing at all.
All this is predicated on the caveat, of course, that I don't know how well Taiwanese pilots are trained, and I do not know what quality of electronics or missiles these Harriers will be equipped with.