There's a basic premise in kick scooter design that says that a lower deck is a better deck. This is because in the act of kicking to propel the scooter, the rider must bend the knee of the leg standing on the deck to allow the foot of the kicking leg to contact the ground. The higher the deck, the more the knee must bend and the quicker the rider fatigues. New scooter riders quickly learn that it's not the kicking leg that tires, it's the standing leg. This is why switching off which leg is kicking and which is standing is such an essential skill for a scooter rider, but that is a subject for another post.
A rider tends not to kick off flat-footed but rather points the kicking toe downward. Taller riders tend to have longer feet and this means in general that they are less sensitive to deck height than shorter riders. Also, riders with more leg strength, such as those with backgrounds in cycling or running, might not be as sensitive to deck height issues as some others, but in general a wise scooter designer will opt for the lowest deck possible.
This limit of how low you can go is determined by ground clearance and material strength. If we all scootered on perfectly smooth, glass-like surfaces we'd only need a few millimetres of ground clearance. And if our scooters were made of NASA-level titanium and carbon fibre they might be equipped with wonderfully thin frames and decks. Alas, we live and scooter in a world with root-heaved paths and sidewalks and our budgets tend to dictate that our machines be made of rubber, steel and aluminium.
My SwiftyONE has a deck height of 105 millimetres and a ground clearance of 63 millimetres. At 5'6" (1.68 metres), I'm not a particularly tall man (my Euro shoe size is 43) I find the Swifty's deck height to be "just right" for me. I find it less tiring to ride than my other scooters that have lower decks, but that is probably due in large part to the Swifty's efficient 16" wheels. And the ground clearance is most welcome because I tend to scoot on some far from perfect surfaces.
I recently let a much taller fellow borrow my scooter as he was looking for a replacement for a scooter he found too low for his Seattle commute. We were able to set the Swifty's handlebars high enough for him. The test ride convinced him and now he's saving up for a Swifty.
Deck height certainly matters, but just as a deck can be too high, it can also be too low. Scooter designers, builders and buyers all have to find out what works for them.