I look at a mixtape as a kind of movie without pictures. It should ebb and flow the way a movie does, with the talky parts balanced by the action. And like a great movie, it should contain two or three strong, memorable sequences around which the rest revolves.
Sometimes a mixtape (or a movie) is born from the desire to create just such a sequence. You fall in love with the alchemy between a series of songs, decide they’d be perfect about three quarters of the way through Side B, then seek out filler to occupy the rest of the space. That might not be the ideal way to create your masterpiece, but it happens that way quite a lot.