Team Rocket's Pokemon are strong! HAHAHAHA! Oh wait you're serious. Let me laugh even harder. HAHAHAHAHA.
Really it's really hard to tell how strong Mimikyu and Mereanie are because they barely appear in the series. Even though they have Z move at their call, they are not really using them. If Team Rocket trained their Pokemon instead of beveling they can win without that. They are not gonna win or steal anything at all. This is why I think Team Rocket in XY are better because they don't have a short less on them.
Not that they are strong but they are in an awkward position where they are actually trying harder to get stronger than most of the twerps but they are still required to get thrashed by them, so they have to mishandle their better resources. By all logic they could likely just have Mareanie use a Black Hole Eclipse on anyone but Ash and Kiawe and that'd probably win.
XY at least had the excuse that they hadn't had a change up since the start of the series while the twerps got more and more Pokemon and tune ups, of course they'd naturally get surpassed, they actually lampshaded how futile it was by the time Ash had become a LEAGUE FINALIST. I mean that still makes them absolutely pointless as antagonists, but it makes some reason WHY they are pointless.
In SM most of the twerps are stagnant for the large part, ironically the only one that regularly gets team changes is the only guy they manage to remotely put on the ropes at times. Hard work hardly works indeed.