A recent post on r/science points to a plague called "cocoliztli" as a major cause of Aztec Empire's downfall, are there any historical records from either Spanish or Meso-american sources that mention this or where it might have come from?

As u/ParallelPain has noted, the post you mention is not connected to the end of the Aztec/Mexica's downfall, which was connected to the smallpox and military conquest (among other things). Rather the plague then called "cocliztli" was the first major epidemic after the conquest in large parts of Mexico, between 1545–1550, wiping out somewhere around 80% of the native population there. Another huge plague came in the 1570s in once again killed about half of the remaining native population in those regions (iirc the numbers from this paper).

As to contemporary sources mentioning this epidemic from the 1540s I can think of two native authors mentioning it:* Domingo Chimalpahin* and Juan Buenaventura Zapata y Mendoza. I'm less familiar with Spanish sources for this time-frame (Franciscans most probably for this time-frame), so this is by no means a complete run-down.

Chimalpahin was a lesser noble from the Chalco province, who lived most of his life in Mexico City. He worked as a copyist for the Convent of San Antonio de Abad under the Capucins, until it was taken over by another order. In parallel to this work he also produced the largest corpus we know of any author in Nahuatl: Mostly historical writings on pre-colonial Chalco, the Mexica and other population groups; but also copies of Spanish authors like Gómara. Less typically, we also have an annal from him written about his own time when living in Mexico City, later simply called the "Diario" (diary), written sometime in the early/mid-17 th century. The pre-colonial annals genre noted important events by year, focusing mostly on the deeds of rulers and nobles. Here's what Chimalpahin notes in it for 1545, in an unadorned writing style:

And also, it was 64 years ago, in the year 1 House [Nahua date], 1545, that blood came out of people's noses. In this year the San Hipólito market was set up. That is how many years it has been until now at the end of this year of our lord 1608. (Here from the English translation called "Annals of his time", transl. Lockhart/Schroeder/Namala)

On to Zapata y Mendoza: He was a noble from Tlaxcala, who was active in the Indian City council for decades in various positions. The Tlaxcalans had been enemies of the Aztec/Mexica, but had profited from being major allies of the Spaniards, and lobbying ot receive special rights in the early/mid 16th c. However, by the later 16th c. Tlaxcala's special status (e.g. with Spaniards officially not allowed in the region) was becoming gradually undermined, and Spaniards started having more political and economic control. Zapata y Mendoza reflects on such changes in his Historia cronológica de la Noble Ciudad de Tlaxcala, written between 1662 and 1692, so even later than Chimalpahin. he wrote in Nahuatl and in the annals form as well.

As the noble's focus is on native administrators from the city council, he remarks on how many of them died due to the pestilence, with some only serving parts of their terms. He also notes already in 1544 how there had been a major draught in Tlaxcala. He says for '45 that "in this [year] came the great epidemic" (my transl.). Another native author added later here that "The Pestilence [was] in New Spain.". For '47 Zapata y Mendoza writes of Hernan Cortés' death.

It's interesting to note here that while Chimalpahin he does not talk of the next major epidemic from 1576-78, Zapata y Mendoza says that in 1576 "there occured a great epidemic". However I'm not sure if this could mean that this later epidemic was stronger in Tlaxcala than In Mexico City - or if it simply points to the two authors different viewpoints.

I'd also like to highlight regarding the very "matter-of-fact" tone of both passages, that both authors started writing in much more detail in those works only when talking about events that had happened in their own lives. In the end, we don't get any attempts at explanations for the epidemic by these authors, but rather an idea of the scale and horror of the epidemic. Then again, it might be improbalbe to expect a (modern) medical explanation from these authors lacking a medical education and writing in the 17th century.

[Edit update:] For comparison I now looked into two other anonymous annals from Puebla and Tlaxcala from the 17th century (translated by Camilla Townsend, as "Here in this year"). Once more, the epidemic of the 1540s is mentioned very briefly here, with the Puebla annal claiming that "800.000 indigenous people died of it in all the realm." Which is of course far below the real numbers of casualties, but tells us something of the estimates at the time. For Tlaxcala there is once more mentioned a famine in the year 1544, which might be in some way connected to the epidemic Charles Gibson (In "The Aztecs Under Spanish Rule", p. 136) also mentions native Tlaxcalan writers talking about a thousand people dying in Tlaxcala every day at that time.

There are naturally more descriptions of the later huge epidemic of 1576. I'll quote two excerpts on this here from Spanish writers, in translations from "David Petriello, Bacteria and Bayonets: The Impact of Disease in American Military History".

The first description comes from the Franciscan Juan de Torquemada, whose "Monarquia Indiana" was a major source on native Mexican history and cultures, with breaks up into the 19th century. He was head master of the Colegio de Santa Cruz Tlatelolco near Mexico City in the early 17th century, and would read about the earlier outbreak in writings of franciscans who had witnessed it in the Colegio.

In the year 1576 a great mortality and pestilence that lasted for more than a year overcame the Indians. It was so big that it ruined and destroyed almost the entire land. The place we know as New Spain was almost empty. It was thing of great bewilderment to see the people die. Many were dead and others almost dead, and nobody had the health or strength to help the disease[d] or bury the dead. In the cities and large towns, big ditches were dug, and from morning to sunset the priests did nothing else but carry the dead bodies and throw them into the ditches without any of the solemnity usually reserved for the dead, because the time did not allow otherwise. At night they covered the ditches with dirt ...

It lasted for one and a half years, and with great excess in the number of deaths. After the murderous epidemic, the viceroy Marin Enriquez wanted to know the number of missing people on New Spain. After searching in towns and neighborhoods it was found that the number of deaths was more than two million.

The other interesting source I found for this is Dr. Francisco Hernandez, mentioned already by u/asdjk482, who was the protomedico or "physician-in-chief" of New Spain and former physician of King Phillip II of Spain. This background explains his far more medically detailed account compared to the other sources I quoted:

The fevers were contagious, burning, and continuous, all of them pestilential, in most part lethal. The tongue was dry and black. Enormous thirst. Urine of the colors sea-green, vegetal green, and black sometimes passing from the greenish color to the pale. Pulse was frequent, fast, small and weak -- sometimes even null. The eyes and the whole body were yellow. This stage was followed by delirium and seizures. Then, hard and painful nodules appeared behind one or both ears along with heartache, chest pain, abdominal pain, tremor, great anxietyn and dysentery. The blood that flowed when cutting a vein had a green color or was very pale [and] dry ...

In some cases gangrene ... invaded the lips, pudendal [genital] regions, and other regions of the body with putrefact members. Blood flowed from the ears and in many cases blood truly gushed from the nose. Of those with recurring disease, almost none was saved. Many were saved if the flux of blood through the nose was stopped in time; the rest dies. Those attacked by dysentery were usually saved if they complied with the medication. The abscesses behind the ears were not lethal. If somehow their size was reduced either by spontanous maturation or given exit by perforation with cauteries, the liquid part of the blood flowed or the pus was eliminated; and with it the cause of the disease was also eliminated, as as the case of those with abundant and pale urine.

At autopsy, the liver was greatly enlarged. The heart was black, first draining a yellowish liquid and then black blood. The spleen and lungs were black and semi-putrefacted ... the abdomen dry. The rest of the body, anywhere it was cut, was extremely pale. This epidemic attacked mainly young people and seldom elder ones. Even if old people were affected they were able to overcome the disease and save their lives.

I don't know enough about medicine to comment on this description. Petriello argues from it and from the fact that it struck mostly younger people that this specific outbreak might have been "something unique to the New World and more reminiscent of the Spanish Influenza outbreak in its fatality characteristic", possibly a tropical hemorrhagic fever. What is more, Adnan Qureshi mentions that mostly native communities were once more victims of this epidemic, which he connects to their poverty, malnourishment and stark working conditions compared to the Spanish population - in contrast he thinks the Spaniards' immunity would have played a less signifact role here than in earlier outbreaks like the one from the 1540s discussed above (in "Ebola Virus Disease: From Origin to Outbreak").

Edit: Added one last paragraph on the epidemic of the 1540s; and a second part on the epidemic of the 1570s.