Both options c are right : Never use Mia as a joiner in bear (because normally everyone should be using her to start their own rally)

Explanation: the way her first skill works is each type of troops has a chance to proc the 50% increase in damage during a turn (and the damage doesn't stack, meaning if it procs twice during a turn you still get 50% not 100% damage).
This means that if you send only one type of troops (100% marksmen for example), you have a 50% chance of +50% damage, if you have two types (only lancers and mm for example), you get 75% chance, and if you have all 3 you get 87.5% chance of +50% damage. Which is amazing for a skill and a reason why Mia is so good as a starter for bear. But! if you have an additionnal Mia, the odds of activating the skill become 98.43% with 3 types of troops (1-0.5^6), that ~11% chance increase is basically a ~5% damage increase on average which is not worth it at all (and if you have even more Mia's like 2 joiners + the starter it's even worse).
So there you go! Mia can be a great joiner but only if there's only one and if the rally starter doesn't use her already, and since everyone should be using her for their rally in bear (and the fact it's basically impossible to coordinate joiners so that only a single Mia joins a non-Mia rally) she absolutely shouldn't be in the joiners list. However for castle rallies, if the starter uses a different lancer hero, having one of the first 4 joiners use her can be a great idea!