For clarification, your opponent will still gain a mana crystal at the start of their turn as well, so this basically cancels the mana growth each turn. So it isn't effective by the time your opponent is at ten mana (since they'll go down to nine mana crystals, then gain one back up to ten).
Mariska is meant to be a control/midrange annoyance card, but it does very little against aggro. The four health makes it so Mariska requires an investment to remove, but not impossible by the time she comes onto the board. You won't be attacking with her very often, so the 2 attack reflects that. I thought a lot about mana cost, but thought that five was a good spot since she couldn't be pulled by most Recruit cards, yet you could still play her in the mid game to disrupt any large costed spells/minions your opponents would be playing. The earliest you could play her (at the time of this post) would be turn two as either Druid or Rogue, assuming you go second and use the coin plus two counterfeit coins/innervates in that turn. Hopefully that would be a fringe case scenario more often than not. :)
Flavor Text: When Refreshment Vendor rotated out of Standard, Mariska had to get her fix elsewhere.