Well...it depends on what Pokemon you're referring to and how you use them. Most Mega Evolutions are like adding nuclear power to a tank, I'll admit; Nintendo seemed to want to kick weaker competitive players by putting in powerful Pokemon like Gengar and Salamence etc. and giving them an even more powerful Mega just to baffle them. And if you're counting the Primal Forms of Kyogre and Groudon and even Rayquaza's ability to Mega Evolve without a species-specific Mega Stone, then yeah, that's worse. Mega Evolutions can be and for the most part are advantages over weaker, overlooked Pokemon that lack them, but that's not really any different from pulling a Legendary Pokemon in the middle of a battle (you know, excluding Kyogre, Groudon, and Rayquaza); they're still beatable, it just requires more roundabout thinking. They have stat advantages and sometimes Ability advantages but they really can't be worse than that random Mew or Deoxys or something.
On the other hand, if you look, there are the underused Pokemon that have gotten a Mega and come back into play, i.e. Beedrill, Glalie, Pidgeot... Upping their power and giving them a new look has conversely imbalanced the first Pokemon to get Mega Evolutions, the Kalos breed, so I don't think that Mega Evolutions have really thrown the game off balance. I mean, yes, there are a lot of Pokemon that shouldn't have a Mega considering how powerful they are, especially the Legendary Pokemon, but in my opinion Mega Evolutions bring new strategic possibilities into play and make the game more fun.