Q: import from iCloud drive

You use the Import command on the File Menu

Well I guess it has not been dumbed down enough since the Import command is under the file menu exactly like iPhoto and most other programs that use imports

A hint - in the Photos Help menu type in Import as a search and it will show you where the command is - this works in nearly every single Mac program

LN

Select the photos in the iCloud Drive window on your iPad and click on the Share button at the bottom of the window:

In the next window scroll the bottom of the window to the left to get to the Save Image button. 

Clicking on that will save the photos to your iPad and put them in the Camera Roll and My Photo Stream (if you don't have the iCloud Photo Library enabled).

NOTE:  these screenshots are from an iPhone 5s and I expect the same workflow would be relevant on an iPad. 

How do I do a simple operation like import the pictures into Photos from iCloud Drive?? There's no "import" button. This app is so dumbed down it's unusable.

It takes a bit to get accustomed to the application centric design of iOS applcations. Apple does everything to hide the file system from the user on the mobile devices. We share from applications to other applications, but we cannot import from files and folders. So share from iCloud Drive to Photos, as described by OT.

As to using iCloud Drive to import photos to your iPad - there are easier ways.  Photos for Mac has been designed to use different iCloud services to import photos.  iCloud Drive is meant to be used for documents, but Photos has its own iCloud services that are easy to use - My Photo Stream, Shared albums, iCloud Photo Library. You can Air Drop photos to your iPad, or mail them, or sync them with iTunes, or import them directly from a card using the camera connection kit.

From any other application use the share menu as shown by OT.

You don't have to do them one at a time. You can select multiple items/photos at one time and do them all at once. Click on the Select button at the top (where is now reads Cancel) and then touch on each item/photo you want to save to your iPhone.  A blue checkmark will appear on those you select:

Are you referring to Photos on your iPhone or your Photos library on your Mac.  Since this topic is in the iPhoto for iOS forum I'm assuming it's in regards to on your iPhone.  When I saved the photos in the iCloud Drive on my iPhone they went into the Camera Roll on the iPhone. 

If you're referring to the iCloud Photo Library I can't help you there as I don't use it.

Don't have an iPad to test on.