That is not an easy question to answer, so lets take it in parts.
First of all for a Level 1 inspection, WETT states the inspector does not need to go onto the roof. But there are times going onto the roof may be beneficial. Having said that, there are potential options where one can do virtually as good a job in many cases from a ladder at the side of the roof or the ground.
For flashing and shell condition often times a good optical assistant will do most of the job, not as good as getting right face to face with the flashing but is fairly close and is all many Home Inspectors would do.
For height measurements can be a bit more challenging. Distance from another part of roof and if chimney needs to be taller? Well for basic height, anybody who has built and worked on chimneys much can tell fairly accurately if it is too short – and if it is close they may need to measure it. I have seen them above roof of cottage only one foot, or sometimes two feet and you can tell from ground easily. Or sometimes it is obvious that it is less than ten feet from peak of roof – maybe six feet as an example and maybe only level with the peak!! It does not take a rocket scientist to determine that is it too short an that it need additional height.
But there are even better options, if one wants to spend some money. There are laser measuring devices which will accurately measure height and you can use to compare to other heights nearby – or they can even measure horizontal distances from afar. How accurate are these devices? Well they are within a few inches for accuracy if you get the professional devices – these units are designed for engineers and surveyors and are not toys. Some will measure distances of up to 1000 metres. No they are not inexpensive but they do the job and do it well. Now if somebody is close they may still wish to get on the roof, if it says the chimney is 2.5 feet above roof.. maybe a trip up would be wise. However if it shows chimney as being two feet too short then there is not really much point in checking on the roof!! Now that is of course assuming one has the tools and I doubt may inspectors carry around $1000 or $2000 measuring devices.
So it is not an easy answer and it depends. But in theory, yes an inspector can do much of the work from the ground and do it as well as somebody who climbs a ladder in many cases if they have the right tools.