Is it sexist to call a woman "cupcake"

1) No, but some women will have you believe most words/ actions are sexist so you're only going to get a mix of responses.

2) Why on earth does it matter if others think it's sexist? It's clearly not, a cupcake does not give birth nor have feminine attributes so I don't see how it can be considered sexist. You're not saying "all women are cupcakes because x,y,z" that might be sexist.

The amount of times I meet a new girl and randomly in the conversation after I say something sarcastic like "I'm not very manly" or "that's a bit girly" and they suddenly change tone and respond with "gosh that's sexist".... Just want to slap them and leave. Can't be coping with people like that lmao.

To conclude, pretty much nothing is sexist, unless you're literally saying "ALL *insert gender* are *insert description* because *insert stereotypical belief about said gender*" and yet still there's no harm done there. Saying something "sexist" doesn't matter, all that matters is if you act on these "sexist" things.

Like for your example, if you literally treated her like a cupcake.

Could this considered sexist or just sweet?

I think a cupcake is rather feminine

These folds indeed remind me something feminine - and tasty.