I’m a general, not some opera floozy!
Isn’t it so gracious that Celes “agrees” to play Maria in the Opera Maria and Draco? Even better, Celes looks like Maria. For all we know, they could have been twins, separated from birth.

Yeah, OK. Sure…
Everything seems to be too much of a coincidence. It’s one thing for her to look like Maria, but Celes also sings like Maria. Even if they were twins, what are the odds that they both took opera lessons? Opera isn’t something anyone can just improvise. It takes years of practice, perhaps a decade at the least. So then, how was Celes able to belt out notes for an opera? I don’t think the Impressario has low standards. And I also don’t think Celes’ genetic experimentation is the cause of her singing prowess.
Maybe the Gestahlian army had an imperial choir? This could explain how she would have had enough time to practice her singing skills. This is common for most militaries around the world. Some of them have Opera singers. However, the Gestahlian Empire doesn’t seem to be a very cultured governmental organization. And Celes doesn’t seem as if she would have time to participate. Since from birth, the Gestahlian Empire groomed Celes to be an imperial soldier. So then it raises the questions: how does Celes know how to sing opera?
She probably had to go AWOL several times over to get that voice. It’s not uncommon for military leaders to be in many places at once, at least in terms of Final Fantasy. Consider Titus Drautos, who was both the military commander for Insomnia and General Glauca of Niflehim. How? In Kingsglaive, it’s not clear. In Final Fantasy VI, we have one explanation: magic. Celes used Teleport to enable Locke to escape Cid’s Lab. It’s not a stretch to say that she used Teleport to escape the harsh life of being an imperial soldier.
And every time, she teleports to the Opera House, under the guise of Maria. Obviously, she has a beautiful face, of which the Impressario noticed as “star material” right away. As for her voice, it probably needed work. So she took lessons, at the Opera House, to build Maria into the star she has become.

Celes must take this disguise; they’ll reprimand her harshly if they discover her. As time went on, Celes became disillusioned with the Gestahlian Empire. She further shirked her duties, putting more of her time into learning Opera. However, one day upon returning, the army apprehended her for going AWOL, during wartime. Right around the time of her imprisonment, the Gestahlian Army was at war with Doma. And with their top general missing, who knows what secrets she might have given away.
We can assume that she spoke out against Kefka poisoning Doma. However, speaking out would lead to a dressing down, not execution. Going AWOL during wartime is tantamount to desertion, punishable by death. This is probably why we meet Celes in the Dungeon.
Yet, all of her practice pays off. When she finally has her big day in the spotlight, she sings with all her heart. And it leads to the memorable scene we see today.