What does and does not constitute a 'real word' is often times up for debate. If language is considered descriptively, than a real word is anything that one person says and another understands. some good examples of this are irregardless (a word stuffy old men often cringe at because they would more then like be considering language prescriptively - which is the school of thought that tries to tell people how to use language.)
Long story short, they both are words, they both have the same meaning of being the highest grade of stupid; however, most is considered a superlative word to be used with ungreadable object like pregnant. In English, pregnant is something you are; therefore, you can not be more or less pregnant.
Another interesting thing to note is:
mass nouns like most: most salt/most rice instead of much salt/much rice (my favorite example of this is at the grocery store 15 items of less - should be 15 items or fewer since the items can be counted, making the word less for mass nouns. So what language should be and how language actually words is interesting, this means if the stupidity can be counted weighed or measured like with other non mass nouns or countable nouns, most should be used, if the stupidity can't be measures or counted much should be used; however, no one thinks of these thinks when speaking (except grammarians and stuffy old men).
If i were to say joetreepop this would not be a word because no one knows what it means. Grammatical leveling is a common feature of language which makes irregular words lake eat and ate more standard: eat and *eated; ox and oxen vs ox and *oxes; fish and fish vs fish and *fishes (which should not be a real word but does not appear to have a red line under it).
in a hurry no time to edit or rearrange ideas. take it or leave it:)