I don't see where you guys are coming from...Mari Iijima and Chie Kajiura are both very talented and expressive singers.
I've said it before, I'll say it again. I believe Macross has been very lucky in the muscians they've gotten over the years. I'm not absolutely crazy about everyone, but there hasn't been an unskilled singer in the bunch.
Except Exsedol. And Gigil. And Gamlin. And Geperunichi. And Fire Bomber American.
And if Leon and Grace burst into "What 'bout my star? @VAJRA HIVE" in the final episode of Frontier, I'll likely curl up into a shivering, tooth-chattering fetal ball for the next few years.
But other than that, I can think of only a few Macross songs that I dislike intensely, and none of them are by Minmay or Mylene (although I admit that their duet, "Friends~Surpassing Time and Space," sounds a little too easy listening for my tastes...).