Are the odds of drawing mythics greater when purchasing individually packaged boosters over booster boxes?
The only differences between single packs out of a booster box at a LGS and blister packs at a big box store are:


The blister packs can't really benefit from mapping1 like the boxes for some sets can.
People can more easily steal cards out of the blister pack and repack them with terrible cards not even in the same set.


Number 2 is especially true if the blister pack comes from Walmart, who have a very generous return policy.

Normal2 MtG packs have a 1/8 chance to contain a mythic (roughly, depending on the size of the set), regardless of where the pack comes from.




"Mapping" a set means determining how the cards in the set are printed and distributed (a process that is generally crowdsourced). Once the set is mapped, one can potentially open a small number of boosters from a box and from there determine what every rare in the box is, and which packs have them. Mapping does not generally cover foils, however. Wizards has spent great effort to prevent the capability of mapping sets, and these days while it is still possible, it is generally unprofitable.
By "normal," I mean to exclude things like special packs used for prereleases such as the guild-specific packs in the Return to Ravnica block. I am not certain on their mythic distribution, so I'll not comment on them.