I'm going to assume that the meaning of "my thing" is as given in What is the origin of “My thing”.
While I personally feel as if it's more common for each person to only have a single "thing," I imagine some people could have more than one "thing," and refer to them in the plural.
The question as asked is ambiguous because it's not clarifying an underlining assumption. Does the person consider themself to have only one thing—or do they consider themself to have multiple things?
I also find some other aspects of the sentence odd.
First, it's more common to say "(I) like pizza" than it is to say "(I) like pizzas." As far as this food-related word goes, unless somebody is pointing out a specific countable number, it's more often used as a mass noun.
Second, it's more common to have the definite article in front of cosmos than not.
Given all of this (and rephrasing a bit), we now have two possibilities:
(one thing) Pizza, unicorns, and the cosmos are my thing.
(three things) Pizza, unicorns, and the cosmos are my things.
Depending on how many "things" we have, both sentences are correct.
Is it possible for "pizza, unicorns, and the cosmos" to be a single thing?
Let's say that I'm famous for routinely going out to pubs, getting drunk, and passing out. (For the record, I am not . . .) In my mind, and everybody else's mind, these three things are closely related.
Doing that is my thing. What is that? Going out to pubs, getting drunk, and passing out. Therefore, my thing is a particular sequence of events.
From the sentence in the question itself, it seems unlikely that there is any kind of association between pizza, unicorns, and the cosmos. But that's only because no context has been given.
Let's say that, once a week, I order myself some pizza, and then sit down and draw pictures of unicorns while pondering the mysteries of the cosmos. (Again for the record, I do not . . .) As with the other example, doing that is my (single) thing.