I also teach game development courses. While I appreciate what you are attempting to do by teaching "marketing", I would recommend against using ratings from random itch.io players as a metric for assigning bonuses.
First, itch.io is a skewed audience. Look at the kinds of games that are highly ranked in most game jams and you are going to see a strong preference for certain types of games (eg: remakes of classic action/arcade games with innovative twists). One of the world's most profitable games is a social bingo game targeting older women, but I doubt that would do well here. Similarly, "Style Savvy" was a fantastic DS game about fashion, but I doubt it would fair well with itch.io's indie-focused audience. By letting itch's audience be the judge of your students' games "marketability", you are incentivizing your students to only think inside the box of this one specific audience rather than opening them to the possibility of appealing to more diverse backgrounds.
Second, you have no idea what metric an individual stranger is really using. A player may not rate a great game highly simply because the game's genre is unappealing to that player; so even if the game were the pinnacle of that genre, it wouldn't fair well.
And, finally, think about how many revered artists today were reviled in their own time. Adherence to sound design principles and intentionality is a good way to judge art because it is timeless. Judging art based on popularity is fleeting and may squash a future revered artist who is simply misunderstood by the masses of today.
If you want to teach your students how to design to appeal to an audience, why not pick a diverse handful of specific people to "judge" their games. Give the students a list of each judge's preferences and let the students design a game targeting 1 specific judge (their choice) or a have the students create a different game for each judge. This would not only encourage learning both good design principles and audience targeting, but it would also present an opportunity to teach about issues like accessibility - especially if you pick judges with special needs (like a colorblind judge or an elderly judge with arthritis).