Is wearing a lot of clothes to conceal body thinness considered forbidden, or an alteration of God's creation, or a sign of not being content with God's decree?

There is no Shari'ah impediment to a thin person wearing several layers of clothing to adorn themselves and cover their thinness. This does not constitute altering Allah's creation, and Allah Almighty has bestowed upon His servants adornment and clothing. The default is the permissibility of any attire unless it involves deception or fraud, for the Prophet, peace and blessings be upon him, warned against deception by saying: "Whoever deceives is not of us."