Sass is fun. Sass is spicy, tingly-hot, and sparkly and it gets the brain revving in a rather mischievous gear. A relatively safe way to write sass is to bash at common inconveniences or threats in life, something like
"Yes of course it's raining, we hit our sunshine quota this week."
The more you point your character's sass at other characters, the more you have to worry about the characters involved, their sensibilities, and, to a lesser extent, the sensibilities of your audience (as in the IRL audience). That latter one is more negotiable though especially if your world operates using differing cultures (in which case, awesome, play around with that!)
A very safe method of being sassy is to diss yourself. It only really becomes a problem if the sassy character sasses themself over something a character within earshot is having genuine problems with. (We audience members otoh can shove it. I'm not about to lose my shit if a cis woman mouths off about how awful being a woman is, for example. This despite me, you know, being a woman.)
If you're in combat with another character, then you're already trying to kill each other so being a jackass or not is a moot point. Feel free to go for maximum insults there. (Or if you must be more realistic, ditch the sass and focus only on killkillkillmaimmaimmaimsurvivesurvivesurvive, but I like combat banter so shush)
I agree with everyone here about respecting boundaries, and if you need a trashy JRPG way to go about it, imagine Sass like a spell, that costs SP. The more SP you put into it, the sassier you are, basically. Each character has their own... SR, or Sass Resistance. The goal of your Sassy Character (SC) is to throw as much sass (SP) at characters as possible, but not going over their SR. The finer detailing of this abstraction is irrelevant, just as long as you have an idea how everything works and puts together. Also don't use this idiom verbatim unless you're in an isekai or people are going to wonder what the fuck you're talking about because levels aren't actual tangible constructs. You have been warned ;)