I know I am going to be sorry for this,
but
rav moshe zatsal wrote that bas mitsva is not seudas mitsvah.
not because it is any less worth celebrating her new status, but because there are certain guidelines as to what costitutes a seudas mitsvah and this doesn’t meet those guidelines.
interestingly,
the debreciner, in his tshuva sefer be’er moshe
comes to the same conclusion. (he apparently was not aware of rav moshe’s tshuva but decided this on his own.)
he writes that when asked, his first reaction was, “Why not. what’s the difference?”, but after looking into the matter, he concluded it was not a seudas mitsvah.
over and out.