I have just imported some pictures from my iMac into a trial version of Lr. Unfortunately the pictures are not at all sharp after importing them. I have to make them very small to get them sharp. But then I have a tiny picture on my screen and that's not workable. Quite a disapointment. Has anyone a clou?
First, your screenshot is not from "Lightroom Classic", which is the subject of this particular forum. It is probably from "Lightroom" - which is a different product. However, these two work in a very similar way when it comes to the steps involved in editing a photo.
Next, normally images that come directly from a digital camera will have lots of pixels and lots of picture detail. Even if this picture detail is not being presented in a very (subjectively) crisp and sharpened way, it is still there. Sometimes a photo may be taken very out-of-focus or with lots of camera shake, but that does not seem to be what is happening here.
I wonder if you have imported a low resolution preview thumbnail of some sort, and not the full original photo that the camera took? Here the particular "route" that the photo has taken from taking the picture, to coming into Lightroom, is important
Also I understand (though I don't know a lot about these, sorry) there are various special image formats within the MacOS / iOS world, different than the industry standard JPG and Raw formats which Lightroom and LrClassic normally deal with. HEIC, or an image with embedded depth information, or Live embedded alternative image frames, or whatever. In the case that Lightroom is being given something special of this sort, it may be unable to do anything except extract a little preview thumbnail and use that.
As a test, I suggest you find a completely standard straight-out-of-camera JPG image somewhere (for example download a sample review image from a camera review website) and import that, and see what the difference is.