It would help to see the chapter / paragraphs that prompted the criticism. It’s hard to generalize, but too much passive voice can be a sign of something else wrong with the writing: a rambling style, lack of focus, over-reliance on dry exposition (“tell”) and info dumps, or purple prose. Using the active voice is often the better choice simply because it makes for crisper, more compelling prose, and this applies to dialogue too. If you’re looking for examples, this site’s very own Ann Everett wrote a terrific book on using active verbs and she’s the real deal when it comes to professional writing.
It’s obviously impossible to avoid using the passive voice altogether, but I think authors also run into problems when they string together long, passive chains. That makes for boring reading, and while it’s hard to put strict rules to it, you certainly know it when you start skimming.
If anything, dialogue needs to be even more purposeful and direct than the descriptive prose you may use for scene setting, character descriptions or world building. A character may use convoluted, rambling speech patterns or drone on with tedious exposition. You could argue “well, that’s supposed to be their personality.” But we’re still reading the dialogue and getting bored, so the end result is the same: we stop reading or skim. If one of your characters is a bore, then why give them precious time under the spotlight?
None of these comments pertain to your writing, of course! I’d have to read the chapter in question to see if I agree with the criticism. If you post a sample here or point me to the chapter, I’ll be happy to give you one novice writer’s opinion.