AirDrop works for this, at least on iOS 17.4.1, where I just tested it. Either aldude's answer is outdated, or he missed a step:
Drag the *.crt
file from Finder on a Mac to the AirDrop item in the sidebar, wait for the iOS device's icon to appear, then drop it there.
If you have an Apple Watch paired with your iOS device, it will ask where you want the certificate installed. After making your selection, you should see this popup:
In Settings, you should find a new "Profile Downloaded" item near the top, under your iCloud info. Click it, then click "Install" in the upper right corner of the resulting screen.
Here's the tricky step: At this point, iOS knows about the CA certificate, but it doesn't yet trust it! You have to go into Settings → General → About, then scroll all the way down to the bottom to find the "Certificate Trust Settings" item. There you will find your newly installed CA under the "Enable Full Trust for Root Certificates" heading, with a toggle button to the right. Enable it.
They couldn't make it easy, could they? 😛