If you're trying to use whore/slut in the prejorative sense, they might actually be quite... um, potent on a goddess, because they were originally used to place a ceiling on a woman's status. Being kicked down from "lady" or "respectable woman" is bad, but when you level that at a GODDESS? Yeah it hurts. I get why you wouldn't want to though, and honestly more power to you for it.
Now for actually being helpful.
What era is your story set in? If your world has simulation games, especially tabletop and video games, you can use slang/lingo from there to draw a line for mockery. Stuff like "Wannabe Dungeon Mistress", or "Lazy Sims Player."
A little farther back, near Greek Plays/etc, if you needed to, something like "Broken Machina," not only implying she's a mere Machine rather than a Person, let alone a Goddess, but a defunct one at that, saying that Greek Playwrights made better "Gods" than her.
In writing, you can get a lot of mileage by not capitalizing her name and pronouns. Most Goddesses demand that you elevate Them to higher standards by specifically writing their Pronouns with the first letter Capitalized. Reference every time YHWH is mentioned in any Testament.
From the Deus-Ex-Machina angle, you could also flip the script and call her Diabolica, or something similar, but this would be less LOL I INSULT YOU and more GRR I HATE YOUR VERY EXISTENCE. Diabolus-Ex-Machina is STILL a cosmically powerful phenomenon, it's just BAD. Which is probably exactly what she intended to do to your MC in the first place.
If she really really likes being a Goddess, and you wanted yet another infuriatingly painful barb? Refer to her in the masculine. "You are a wonderful God, My Lord," and the like. This will probably piss her the FUCK off if she cares in any way about being a She*. (I would know, I deal with that shit IRL.) This could get you in scalding water with respect to the trans community though, and you will certainly have to be mindful of the Terms of Service: "May create a risk of harm, loss, ... mental injury, emotional distress, ... or mental illness to you, to any other person, or to any animal;" and "Contains any information or content that we deem to be unlawful, harmful, abusive, ... harassing, ... threatening, or otherwise objectionable;". I won't bitch at you if you go this route; I mean, you have this human fighting a fucking goddess, of course they're going to fight dirty. This will just paint your MC as being grey instead of puregood.
Oh yeah, that playing dirty thing: Humans fighting fair against Deities, lose. If your Goddess has a "worship day" (let's say Saturday), do your attacks then, and not during her "business hours". If your goddess is visibly asleep, attack then. If your Deity is in the middle of some super important concentration thingy, throw your caustic insults then. If you are fighting a goddess, the playing field is a 90-degree WALL. EVERYTHING your MC does to deal with this Goddess needs to focus on de-slanting that wall to make fighting her even possible. Carry this mentality to your insults as well.
And that includes, every once in a while, addressing her correctly, and with the reverence a goddess deserves. That ought to stall her goddess brain for a few seconds.
*(If you write your MC as trying this and the Goddess is like "you think I give a fuck about such a mortal trifle as gender? HA HA HA", let me know and I will favorite that chapter right then and there.)
