Understanding the game mechanics:

The Counter
The number in the top center counts upward for each consumable that spawns, reaching a maximum allowed of 20. Any drop afterwards leads to a loss. Collecting objects with the Left Mouse Button reduces this counter regardless of the object.

The Points
The number in the top left corner increases when consuming the drops but decreases when consuming mushrooms.

Vegetables/Fruits
With the exception of mushrooms, which last a few seconds, all other fruits and vegetables persist permanently until consumed.

To unlock the XXX achievement, you'll need to score 999. To new players, this will seem almost impossible until you combine the game mechanics mentioned above:

The store description incorrectly recommends avoiding mushrooms while consuming other items. Since the counter goes up for all spawned items including mushrooms and mushrooms despawn if left uncollected, you'll eventually bleed points on the counter that you can never get back. This makes it harder (or impossible) to maintain the counter below 20. Luckily the game is somewhat forgiving and you can miss quite a few mushrooms before it becomes difficult to maintain the counter in a comfortable range.

Understanding this, the objective of the game becomes almost entirely about mushroom collecting. With this strategy getting to 999 becomes very doable with reliably consistent results.

The Strategy [In descending priority]

1. Collect mushrooms
   
   • Practice keeping an eye on mushroom drops
     
   • At any point there are only two mushrooms due to despawn
     
   • Most times their drop locations are close together
     
   • You can reasonably ignore a mushroom once in a while and focus on collecting other foods to bring the counter down to a reasonable range if you're falling behind
2. Collect The Other Items
   
   • Mushrooms take a second to drop and last a few seconds, use this time to collect other foods
     
   • Collect other foods along your route to the next mushroom
     
   • Collect other foods in nearby corners in between mushroom drops
     
   • It's best to stop just short of the food when trying to collect it. Bumping into it glitches the pick up and can throw the bird over the food thus wasting time
3. Pause
   
   • If you are feeling tired and your counter is within a comfortable range, feel free to pause the game from the top right corner to recover