Yes you need Med armor proficiency to wear Med mithral armor. It counts as light armor for ever other reason though. This was a change from 3.5 where is counted as light for everything even proficiency.
While I don't personally think it was a needed change I see why they did it. Being able to use light armor pretty much always meant that C would end up in Mithral breastplate, If everyone uses said option then maybe its a bit to good? Granted on the grand scheme of things there are far bigger issues. Still I like that it is now a choice that needs thought about and the cost weighed against the benefit.
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If you don't mind a little min-maxing, grab one of the traits that lowers your armor check penalty by 1. I see it all the time on PFS characters that only get Light Armor proficiency but really want a Mithral Breastplate. If you're big into math, a trait is normally treated as half a feat, but in this limited case it's as useful as the Medium Armor Proficiency feat and some other feat that gives you +1 AC, since a Mithral Breastplate has a total Armor Class bonus one higher than a Mithral Chain Shirt. That's a 400% return on investment.
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Or you could try this armor. Wont need to spend a feat with this one.
Elven Chain
Aura no aura (non-magical); CL —; Weight 20 lbs.; Price 5,150 gp
DESCRIPTION
This extremely light chainmail is made of very fine mithral links. This armor is treated, in all ways, like light armor, including when determining proficiency. The armor has an arcane spell failure chance of 20%, a maximum Dexterity bonus of +4, and an armor check penalty of –2.
Problem here is that it's an "as-is"-item. The intention is that you can't alter/customize it as with the generic [i]+1 mithral chainmail[i] (or whatever).
Ruyan.
note that if you are say a magus with medium armpr prof and casting you can wear a mitral hell knight full plate if you have the armor expert trait and spend a little gold each day on armor ointment...
You could also wear a Mithril Kikko. Who cares about proficiency when there is no armor check penalty on the armor.
Bards do, as does anyone else who needs to be wearing light armor to avoid spell failure.
Ah, but mithril armor counts as light armor for everything except proficiency, so no arcane spell failure chance, whether or not you are proficient with the armor. So a summoner or bard can use mithril medium armor with no ASFC, but might still take the ACP to attack rolls without something like armor expert.