My theory on the different appearances of the Klingons is based on the idea that there is a "Temporal Cold War", where various factions muck up the timeline to their advantage, as established in Season 2 of SNW. So, in order, the Klingons developed from Proto-Klingons to DISC Klingons to TOS Klingons to TMP Klingons and finally to TNG Klingons. We're not looking at just different types of Klingons; we're looking at the evolutions (literal and figuratively) of the Klingons. But because the timeline is being mucked up IN-universe, we, as the audience, see the discrepancies literally out of order. So, any Klingon's physical appearance is what they have always looked like, from a certain point of view. And, as alluded to by these different quantum realities, which all exist within the same timeline (think of strands of a very large rope, with each strand being on a different schedule of time). So, a DISC Klingon's appearance could easily be around the year 2360s in one reality, where that same appearance could exist circa 2460s. It's just that such appearance, and its subsequent evolution, have their own time-table of occurring. Another theory of mind is that rather than seeing branching timelines as a tree, see the timelines as a railroad yard, with some tracks branching off before merging again. JMO.