Sorry I'm late, but now I'm here: This gives the indication that it was scrapped from one generation, but added to another. So... Kanto to Johto seems likely.
Based on your knowledge of blobs and mice: Ditto is a blob, and Rattata is a mouse. They're both useless Normal-types, so that could help.
You may assume things about me, but think twice: People think it's another gimmick, but it isn't.
A few years ago, my mom was replaced: Possibly hinting to an artificial Pokemon.
By a killing machine with a smile on its face: There are many possibilities for this. Gengar, Genesect, Metagross...
The mountains have shaped me and I have evolved: This points to an evolved Ground- or Rock-type. Or... a Pokemon that lives in caves!
But I will yet again. Can this riddle be solved?: So, a middle evo. And no, it can't.
EDIT: Ignore my last answer. I think the answer is... Missingno. It just makes sense.