Are most living things made up of mostly carbon dioxide?

No,

Carbon Dioxide is a waste product from energy production (respiration). When dissolved in water it becomes carbonic acid (H2CO3). Humans do have a little dissolved in blood, both as a transport mechanism, and as a pH buffer.

Life is made up with cells consisting of a cell membrane filled with water and various proteins. By varying estimates, humans are about 60% or more water.

The most prevalent elements in the body are Oxygen, Carbon, and Hydrogen, with the greatest number of hydrogen atoms, but since they are so light, less mass due to the hydrogen...

I can see how one could come up with the assumption that with carbon and oxygen being the top two chemicals in the body by weight that it would be carbon dioxide... but it is mostly hydrocarbons and hydrogen-oxygen (water) compounds.