As a DCE replacement is not generally essential, you are giving your opponent more time to lysandre through you and take an initiative. Also, I believe jirachi is generally not found in yveltal lists due to the following sequences:
yveltalEX with (D,DCE); jirachi knocks off dce; yveltalEX attaches (D) for turn and passes where no advantage has been gained.
Fright night hits benched shay for 60; you send in jirachi; your opponent reattaches a 2nd dce and ko’s the shay anyway; stardust again and that makes 2 turns that your opponent can gain initiative through the use of lysandre.
In both scenario’s, if your opponent had lysandre after you stardust; they can profit. In short, Stardust jirachi doesn’t bring any advantage in the mirror.
N.B. winning a losing yveltal mirror is very difficult, using jirachi as a fall back isn’t really a valid strategy.