It is also used in Puerto Rico but there its mostly used to describe someone who is foolishly stuck up or pretentious.
Don't be foolish...
Ese show es una come-mierderia total.
That show is a total stupidity.
Are they stupid or what?
Andrew Dice Clay y Soulja Boy son un par de tremendos come mierdas.
Andrew Dice Clay and Soulja Boy are a pair of tremendous dumb-asses.
Yo usualmente cojo los viernes para comer mierda.
I usually take Fridays to goof off.
2. someone who doesn't know what the hell they are doing, what they are talking about, isn't paying attention to what they are doing, is stupid, etc.
3. someone who has nothing better to do than write hateful shit about their latino brothers (be it against Mexians, Puerto Ricans, Dominicans, Cubans, or any other Latinos) on UrbanDictionary.com. Find something better to do with your time, comemierdas.
(Commonly used Cuban/Puerto Rican(?) expression)
Guy 2: goooood you are such a comemierda.
Guy 1: well at least I have better things to do than write hateful shit about other Latinos on Urban Dictionary. who's a comemierda now, mojon?
Guy 2: ...