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.
I'll hijack this slightly off-topic post to reiterate my argument on armor in BG3: its in-game availability doesn't make sense. We can get +1 studded leather armor and half-plate/+1 scale mail armor, but there is NO splint armor, let alone plate armor in the game! Even Chain Mail is gotten moderately late into EA, forcing heavy armor STR fighters to use Ring Mail or medium armor (which is terrible if you have a negative dex modifier).
Certain classes should be able to start with chain mail, and should get splint armor roughly around the time other classes are acquiring normal studded leather and breastplate (~level 3). Ring mail basically shouldn't exist in the game, as it is worse than the base armor given to heavy armor classes.
So yes, Minthara should certainly not have ring mail. She should have splint armor at the very least, but plate would also be a good option. Even medium armor (scale mail or breastplate) would provide her with more AC because of her dex mod of +1. Defeating her could grant the players their first opportunity to obtain splint or plate armor, which is appropriate for level 4 characters. Plate +1 would make it too difficult to defeat her and is too powerful a reward for lvl 4 players.
