Can RV dispatches reward Prismana eggs?

That is a chance, not a schedule. A dispatch either comes back carrying one or it does not, so the only lever you really have is sending dispatches out whenever they are available and treating a Prismana egg as a bonus rather than something you can plan a roster around.

The egg on its own is not the prize. Prismana eggs can be hatched, and what comes out has the potential to be a perfect Aniimo. Potential is the word to hold on to. The trait sets the ceiling of what the egg can produce, not the floor.

Incubation is worth paying attention to as well. A Sparkle Cube, also called a sparking box, can be applied to certain eggs while they are incubating, so check an egg before you leave it to hatch untouched. Dispatches are also not your only supply of eggs. Global launch rewards hand out eggs including a free Perfect Egg, which means you have something in the incubator without waiting on a dispatch to come good.

Why a Prismana unit is worth the wait

The payoff shows up in what a Prismana Aniimo brings to a party. Prismana Glacie's exclusive skill, Flickering Shot, grants a 15% team damage buff for 15 seconds, and it is unconditional, so there is no setup you have to satisfy before it applies.

That buff lands on the whole team rather than on Glacie alone. A Prismana unit can therefore earn its slot in a party built around somebody else entirely, which is a different kind of value from a single strong attacker.

Not settled: Whether Prismana eggs are among the eggs that accept a Sparkle Cube during incubation is not settled, and the rate at which dispatches return them is not a published figure.