Is saying afters instead of dessert bad English?

"Afters" is not considered standard English for "dessert." It's a regional dialect word primarily used in British English, particularly in some parts of the UK.

While it's perfectly acceptable in those regions, it might sound unusual or even unfamiliar to people from other English-speaking countries.

If you're aiming for clear communication with a broader audience, stick with "dessert" or other more universally understood terms like "sweet," "pudding," or "treat."