You did a really good job in your description of the meaning. Your offered translation captured the essence of what's meant. Props to you for that! Looking for an equivalent, you'll have to avoid formal English, because this isn't formal speech in English nor Chinese.
A good translation would be a "smart-ass comment." That's what we call it in English when someone makes an off-the-cuff remark about something mentioned, usually to poke fun at it, without meaning it to be relevant to what's being said.
Sometimes when we do this in English someone "coughs" the comment to make it more comedic as if slipped into the conversation by accident.
A "roast" is a comedic event where friends of a person stand up and speak badly about him, as a celebration of him. Sounds pretty strange, right? Westerners have tremendously impenetrable egos, so sometimes when we poke fun out of love. Like when we toast our best friend at his wedding, we might ridicule him a little in admiration. That's a roast and I don't think it lines up well with what 吐槽 implies.