The only prerequisite for Plate Armor is Heavy Armor Proficiency and Strength 15. As a Cleric, she should meet both of these. It also makes no sense that a boss villain would have plain Ring Mail armor at all. Why would she NOT have some sort of superior armor? She's a boss. That's when players SHOULD acquire awesome armor, while beating a tough boss bad guy.
The more I look into Minthara, the more disappointed I am with what they have done with her. There's no reason why she should wear plain Ring Mail and carry a plain Shield other than that Larian doesn't want her being too OP for the PC's. BUT, she would make a perfect villain with better gear if the PCs are either able to acquire better gear themselves beforehand OR we are given a party of 6 as opposed to 4. There are ways to make it work besides totally nerfing a potentially cool villain.
