I believe it's in bad taste. We don't make art out of executed criminals. In the same vain, making art out of pests is reprehensible.

Oh, you must be an alien. My species, humanity, has been displaying the corpses of criminals for the entirity of its existence and continues to create paintings, songs and movies about it. The question is more "Is it art, or is it porn?", followed by " Is porn art?". Find me a catholic church which doesn't have a cast crucifix. That the ants were in the 'mold' feels like an implementation detail. They were scheduled for death. Weather the aluminium was poured before or after the extermination feels like an implementation detail. But do I want one? Fuck, no. It was interesting for science, but once was enough.

"Hey, look at those ants on that blue planet. Are you ready to fire the Death Star?" "Armed and ready, planet has gone molten. Awesome, get the contracts ready, I got dibs on the cast of the NYC subway, I heard that those ants really loved those tunnels, man, it's gonna look awesome in my gazeebo."

"They were an invasive species anyway, always scheming to extend beyond their own planet."

Bad taste yes, but bad taste is subjective. Morality (should be) objective, so it's one thing to say "I don't like this" (I don't like it either), and another thing to say "nobody should like this".

Morality isn't black and white. It's somewhat bad to be killing ants but let's not say it's on the same level as killing sentient life. It's just a rather shitty thing to do.

The issue here isn't whether or not to kill ants (we're killing them anyway), it's whether it's immoral to make art out of their nests.

Considering the very thing that is the object of beauty, it seems perverse to have destroyed it and its forebearers in entirety to produce it. Something akin to a buffoon's take on art.