Well, what is "common" to you? A toddler in New York City might know what a bodega is, while you might reach middle age in Baltimore or Salt Lake City and never encounter the word in conversation. And a toddler in Los Angeles might know what it is, but know it as something different from the New Yorker.
Something you can do is estimate prevalence based on samplings of recorded communication (e.g. broadcast media, print publications). There are a number of corpora developed exactly for such purposes. For what it's worth, a Corpus of Contemporary American English search finds these frequencies:
- 17216 minor (as adjective)
- 5123 irrelevant
- 2741 trivial
- 2396 insignificant
- 1673 pointless
- 1225 unimportant
- 1023 not important
- 832 inconsequential
(There are a number of tools for exploring this and other corpora developed by Mark Davies of Brigham Young University which are now available on www.english-corpora.org .)
Now, of course, those words are not indiscriminately interchangeable with trivial. Certainly, minor and pointless have non-synonymous uses which may well be more common. False positives will arise from things like names (for example, the board game Trivial Pursuit). One must furthermore be conscious that a corpus necessarily over-represents some texts and under-represents others, because not all speakers are represented equally in recorded communication. Also, importantly when you are looking for conversational usage, written language tends to lag spoken language, and there have historically been many more collections of written English than transcriptions of spoken English.
Keeping all that in mind, however, I think one could make the case that trivial is not a rare word compared to words which can have similar meaning.
Google NGrams won't do much for you for spoken English, but it provides an easy way to compare the prevalence of words and phrases in the Google Books index visually. As with the other corpora, searching on a phrase to minimize false hits is advisable. Trivial things turns up much more often than minor things or insignificant things.
For a different, less generally applicable proxy, you can try to see if a word appears on some or other vocabulary list for different grade levels. Trivial is suggested for a 7th grade vocabulary list published by Flocabulary (7th grade in the US typically enrolls 12- and 13-year olds); it is on a sixth grade vocabulary list (11- and 12-year olds) published by LearnThat. I do not know anything about these organizations or the quality of their pedagogical materials, but this suggests that trivial is a word that Americans might be expected to learn by the end of middle school, and so a word that would not be unusual in publications such as daily newspapers or mass market magazines.