Just about anything is more acceptable in dialogue. However, you need to be sure that your dialogue is comfortable for the reader -- be sure it flows and comes off the page easily.
Dialogue must mimic real speech, and people do talk in passive voice and fragments.
It's true that people do talk in passive voice and in fragments -- I agree with that. But I disagree that dialogue must mimic real speech. Instead, I think it's better for dialogue to give the impression of real speech while still being pleasant to read.
It's easier to illustrate this than to explain this, so here's a bit of dialogue that mimics real speech:
"Hey."
"Hey."
"Did you see the email John sent me?"
"Yeah, I was like..."
"I know, right?"
"I mean, that was just...god."
"Seriously."
"I was like, whatever."
"Yeah, uh..."
Real dialogue is full of vocal static, and also relies heavily on inflection, facial expression and body language to help round out its full meaning. It's very hard to replicate all that in writing. Whatever is missing from expression and physical gestures usually has to be filled in with speech that is more precise and articulate than most people actually use, and with the internal observations of the POV character. The end result is, hopefully, something that FEELS like a real conversation but that actually conveys more textual information than a real conversation carries.
Dialogue is pretty hard to do well, really.
But yes, you can use passive voice in dialogue. You can use passive voice anywhere, if it works. When you're digesting all these "rules" of writing (no passive voice, no adverbs, etc.), remember that they're not really rules. They're guidelines to help newer writers avoid overdoing the things that new writers commonly overdo. But all of the "rules" can and should be broken when it feels right to do so. You just have to understand why you're using passive voice, or adverbs, or whatever other rule you're breaking, instead of just throwing words at the page without a good reason.