Mobi does not support embedded fonts. Whatever font you see is set by the program used to read. Mobi itself has no control over this. Jimmy's suggestion won't work at all. Are you sure you are not saying the Cacellia fonts looks like a typewriter font? That is the default font on a Kindle. When you go to the AA menu on the PW, you will see that in the choice of fonts.
Go to the thread on fonts for the Kindle and you will learn about other fonts and how to install them on your PW. Won't work on a K3.