When decks are exported to a PowerPoint file, they go through a process where they're basically turned into a series of images. The text on a slide in Haiku Deck is an object that is crisp and visible and can be edited, but everything is turned into an image when exported to PowerPoint, and some quality is lost. You can kind of think of it like cutting out numbers and putting them on top of a sheet of paper, vs. a xerox copy of the two together magnified by the copier to 150% (for lack of a better analogy).
Some image quality is lost in the compression that takes place at that point. The best workaround I can provide would be to take a screenshot of the slide in playback mode (iPad: press both the home and sleep/wake buttons simultaneously to save the image to your Camera Roll), and use it in place of the slide in Keynote or PowerPoint. The PNG saved by the iPad may be clearer than the exported PowerPoint image, since the PowerPoint files exported do compromise some quality to make the file small enough to email gracefully.