Ni modo is a popular Mexican Spanish phrase that expresses resignation, like saying 'Oh well,' 'What can you do?' or 'There's nothing to be done about it.' It's a must-know expression if you want to sound like a native speaker.
In the song, the singer has been caught cheating and says, "Ni modo que me crea que somos amigos" (There's no way she'll believe we're just friends). He uses the phrase to show he's accepted his fate and there's no use in trying to lie his way out of it.
Grupo Frontera kicks off this track right in the middle of a messy disaster: Ya me cachó de cabrón ("She caught me messing around"), and he knows full well there is no way she will buy the classic excuse that he and the other woman are "just friends." The Mexican slang here is blunt and funny. When he admits, Ni modo que me crea que no nos comimos ("No way she believes we didn't eat each other up"), the verb comerse literally means to devour each other, but colloquially means hooking up. Instead of begging for forgiveness, he points right back at his partner. The relationship was already ruined, and he cheated purely for revenge because she stepped out first (Por eso la engañé yo a ella / Pa' que como me dolió, le duela ["That is why I cheated on her / So that it hurts her the way it hurt me"]).
He calls out the double standard directly: Si tú la haces, está bien, si te la hago yo te caga ("If you do it, it's fine; if I do it to you, you get pissed"), neatly labeling the whole disaster as karma. He tells her to drop the sad routine because neither of them is innocent (Aquí no hay víctimas, no te hagas la triste ["There are no victims here, don't act sad"]). By the end, he is so fed up that he tells Cupid to stay far away. Watching someone preach about forever only to cheat before Christmas (ya Navidad llegó ["Christmas has already arrived"]) completely burned him out, leaving him swearing off fuckin' amor for good.