"Show, don't tell" doesn't apply to manga or comic books the same way it applies to standard prose.
On the one hand, because manga and comics are visual media, almost everything in them is inherently shown, even sound effects and dialogue. The only way for a mangaka or comic writer to "tell" the audience something is through narration boxes, which have the additional drawback of taking up valuable space on the page.
On the other hand, there are a lot of things that are hard to show through static images alone, without the benefit of motion or sound. As a result, both manga and comics have a long-standing tradition of having their characters narrate, either out loud or in thought bubbles, what they are doing, what someone else is doing, what's going on around them, and so on. All of this helps the reader understand what's going on.
In the case of your specific example - Enrico Pucci explaining his Stand's powers - this is justified because, from what I know of said powers, they fall under the category of being hard to convey through static images alone, so someone has to explain to the audience what he's actually doing. It's also common enough that TV Tropes (obligatory warning: massive time-sink!) lists it as a trope in its own right: Explaining Your Power to the Enemy.