Overgrowth says to shuffle it into the top 4 cards of the Monster deck. Is the top (face up) card included in the top four (meaning that after shuffling, the Overgrowth could now be the top card, which would then be turned face up), or do you shuffle Overgrowth into the four cards beneath the face up card?
Flip the top card face down, pick up the top four cards, shuffle Overgrowth into them, put them back on top of the monster deck, and turn the top card face up again. So yes, Overgrowth could be the top card.
Note that like finding a Shadowrift with an Explore, the top card won't enter play until the Add New Monsters step, so if an Overgrowth shows up during the Heroes Act step (for instance, because of Wormwood Druid, or because multiple players have Explore) you could Explore it away.