Differentiate the most important and the less important cards.
It would be better if you could grade the importance of a card :
For example : 0 for less important cards, 1 for "normal" cards and 2 for the most important cards.
This way, the cards that would come the more often woud be at the same time the cards you know the less and the most important ones.
This means you learn better the important parts of your lesson.

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Abakus commented
There is a similar feature request out there. If you want to learn the important cards before the unimportant cards you might also like to vote for:
"Make card dependent on other card"-feature: https://mnemosyne.uservoice.com/forums/164265-general/suggestions/3053605-make-one-card-dependent-on-having-successfully-lea