Dialogue on the cover works in some instances. Look at Marvel Comics covers from the 1970s for examples of BAD dialogue on covers. A typical AMAZING SPIDER-MAN cover of the period might have the Bad Guy saying: "Ok, Web-head, I've been itching for a rematch" (next balloon) "And this time" (third balloon) "I'M GONNA WIN!" Then two thought balloons for Spider-Man. "He's not kidding! Once those electric shocks/sharpened blades/acid splashes/whatever hit me --" (Second thought balloon) "I'M FINISHED!!" AMAZING SPIDER-MAN #152 is a Fine/Bad example. Perhaps nothing is inherently wrong with this graphic/copy ... but in hindsight it felt like EVERY Marvel Comic of the 1970s was like this. Almost like Mad Libs. But the AMOUNT of copy leads that werid time discrepancy — how do the Marvel characters manage to address each other during a high-pitched battle? Copy on covers? Keep it light ... or keep it to editorial/narrative captions ... or better yet, DON'T. Look to the classic pulp magazines (like DOC SAVAGE) for examples of powerful images that covey story ideas or details without lots of clutter.
