Like I said in a comment above, FF XII used the word "Mist"... Magicka cannot possibly sound silly compaired to that...
It's a matter of taste. To me magicka is silly because it's "magic" with old-timey spelling and an "a" put on the end. In the FF games they come up with a whole bunch of words for magic ideas and phenomena, often drawing from historical lore.
To me both "magicka" and "mist" are silly, but it's the idea of finding a word to describe something we know and understand from mythology, religion and fiction, and using that to describe the thing in the game that's interesting. We understand what health, stamina and magic are, but we choose to alter the word for magic. This is true in a lot of games.