Is the feeling of beauty without makeup, and refraining from reciting the supplication "O Allah, I seek refuge in You from associating partners with You knowingly, and I seek Your forgiveness for what I do not know," out of fear of falling into ostentation or mockery, a form of ostentation?

It is apparent from your questions that you are suffering from waswasah (obsessive whispering). The cure for this is to turn away from it, pay no attention to it, and stop asking about it, all while supplicating humbly to Allah. Ordinary worldly actions are not subject to the forbidden riya' (showing off). As for seeking forgiveness from a sin while persisting in it being closer to mockery, this meaning has been narrated, but it is not authentically attributed to the Prophet, peace and blessings be upon him.