You are not hypoxic below 96% and lets not confuse medical treatment with normality.
I would expect as a norm to see around 87-90% sat at FL100 for a healthy adult. When you are new to being at altitude and not aclimatised this can lead to feelings opf fatigue and in some not being the sharpest knife in the box.
If you use supplemental oxygen delivered by canula you can expect to improve this to between 95-97%.
A smoker will experience slightly lower levels of O2 as Heamoglobin bonds with CO 25 times more readily than O2 and as such it is more difficult for the O2 to transfer. If you must smoke and fly without O2 try and refrain from doing so for a few hours prior to the flight as it will aid your body in eliminating the CO and make best use of the available O2.
Regular exposure to altitude helps our bodies become acclimatised and transfer the o2 more readily. Its the reason why people who live on the side of a mountain have no problems.