It is one of many phrases that cannot be literally translated to make sense.
Cabrón literally means a big goat, but it is NEVER used in this context. It is traditionally an insult on the level of calling someone a bastard. The bastards of Inglourious Basterds were all great examples of c̫ѧʙʀoɴes. As with many Spanish words, the way it is applied implies the meaning. Cabrón could be an insult (like ßɑꜱʈɑɾɖ or the ugly word for ɑɲυꜱ) or it could just be slang (albeit vulgar) at the “guy” level. What’s up, cabrón? = What’s up, man?
Viva is the subjunctive of vivir which means to live. Literally Viva Mexico means that Mexico lives long.
So the free translation in American English would be “Mexico Forever, MFs!”
Sources): I have lived most of my life in Mexico and I am fluent in Mexican Spanish