I have tested all three of the Kanto Starters in X and Y. All three seem to have been slowed and weakened majorly, especially Blastoise, as mine kept getting knocked out in one shot by Water Types and even Fire Types too. Same thing with the others too. Charizard was great until it Mega Evolved, as for whatever reason, after doing so, it was knocked out in one shot during Sky Battles in X Version. Any help is appreciated. Thank you.
Some older Pokemon had small stat changes, as listed here on Serebii. However that does't include any starters, and they are all stat increases, not decreases.
The second half of your question implies you're talking about Pokemon getting weaker when mega-evolving. As far as I know, no Mega Pokemon get a reduction in stats, but some of the stats stay the same (e.g. Mega Charizard X's special defense). Also remember that Mega Charizard X changes type so maybe you got hit by a Dragon type move and it was super effective.