'Mostest', with a 't', suggests a double superlative...ie the hostess who is absolutely the finest hostess one could ever hope to meet. 'Most' is already as far as the word can go, being one stage further then 'more', so it is just a facetious word-creation suggesting the very very best there is.
It's a bit like saying, "He's still my bestest friend!" If I remember rightly, that's exactly what Prince Andrew's wife, Sarah, said about him shortly after they separated. It's a sort of nursery-speak, suggesting the utterly finest example of something or other.