The short answer is, yes, you can Mega Evolve in this remake. As a remake, it includes features and even Pokemon that weren’t in the original games. Mega Evolution, introduced in Pokemon X & Y, is here as well for those Pokemon that already have a form.
How to Mega Evolve
In order to Mega Evolve in Pokemon Mystery Dungeon: Rescue Team DX, you are going to need to likely beat the main story. To Mega Evolve, you need Empowerment Seeds, and we didn’t find any of these until after beating the game. Though, it is possible that you could find some earlier, it’s just incredibly rare.
Empowerment Seeds are random drops inside dungeons and occasionally mission rewards.
The Empowerment Seed will show up in your inventory with the other seeds like Reviver Seeds, Blast Seeds, and so on. It can be easy to miss it, so always play close attention to what seeds you have.
If you use an Empowerment Seed on a Pokemon with no Mega Evolution, it will simply be in the “Awakened” state with better stats. If you use it on someone like Charizard, though, you will suddenly have a Mega Evolved Pokemon.
The Mega Evolution will last not just for the floor you are on, but for the rest of the dungeon, so be sure to use these at the right moment as they are not the easiest to find. Your best bet is to purchase one in the shop when it becomes available there or find it in the difficult post-game dungeons.
All Mega Evolution Pokemon
Here is the full list of Mega Evolved Pokemon in Mystery Dungeon DX, some of which are actually Starter Pokemon: