Hype game or be modest in game dev tycoon?

What Shadur left in comments is correct. If you are expecting high results on your reviews after development, especially if the game has the chance to be a hit, you want to hype it. It will result in increased marketing and sales/units sold. If your end product gets bad reviews and overall isn't that good you will take a large decrease in sales and your fan-base could decrease by more than average.

I highly recommend goolging the wiki for (can't on this pc [work filters]) and looking into what makes a great game or what causes x and y... ETC.

The best thing to think of when you encounter this is it is a risk/return multiplier. Good turns really good and bad turns really bad.

Also, when you "be modest" and in turn develop a hit game it will not hurt you. You will get a message about it, but no negative impacts occur.