The UCs recently established caps on non-resident undergraduate enrollment. At five campuses (UCD, UCSB, UCSC, UCR, UCM), the cap is 18%. At the other four campuses (UCB, UCLA, UCSD, UCI), the caps are variable because they are based on 2017 enrollment, but range from approximately 19% to 24%.
So no, the UCs are not being overrun by nonresidents, regardless of the high non-resident acceptance rates at some campuses. For the UC system as a whole, non-resident undergraduate enrollment should max out at about 20% (with some campuses slightly higher and others slightly lower). That’s quite low by national standards; I think the only lower systemwide cap is UNC at 18%. UTexas has a non-resident cap of 10% for the flagship Austin campus, but it doesn’t apply to the other campuses in the UT system.
You will notice that the highest non-resident acceptance rates (above 55%) are at UCD, UCSC, UCR, and UCM. These are all campuses that are currently below their designated “caps” for non-resident enrollment, so they can afford to be generous with non-resident admits. If and when they hit their caps, the non-resident acceptance rates are going to fall. This is likely to happen at UCD in the near future.