Are athletic exercises that increase height, for the purpose of improving appearance, considered a change to Allah's creation and thus forbidden, and is supplicating for that considered an objection to Allah's decree?

There is no harm in attempting to increase height through exercise or by supplicating for an improved physique. This is not considered altering Allah's creation, nor is it an transgression in supplication, nor an objection to divine decree and destiny. Nevertheless, a Muslim's greatest concern should be the improvement of their character. The Prophet, peace and blessings be upon him, used to supplicate: "O Allah, You have made my physical form good, so make my character good too." And it is also supplicated: "O Allah, make my inner self better than my outer self, and make my outer self righteous/good."