Okay so the theory that I saw basically was that the Morty waves cancel out the quote unquote genius waves of Ricks because Morty is so stupid he is at exactly the opposite Spectrum as Rick. But I don't believe that's right, we've seen multiple examples of Morty really actually kind of being smart, like in total rickall, it was Morty who figured out how to tell the parasites from real family members, he figured it out even before Rick did. I think the more logical explanation is how they mentioned it in the actual episode, that mortys waves are complementary not opposite. don't get me wrong, Morty is still an idiot most of the time but I don't think he's stupid to the point of being opposite from Rick in that way, I think Morty had it a little bit closer when he asked if it was because their personalities were so different. honestly if anyone was going to have opposingly stupid brain waves to Rick would probably be Jerry. I think it makes a lot more sense for morty to be Rick's counterbalance, more like the yin to his yang, equal but opposite. A Morty wouldn't be able to properly cancel out a Rick if he was inferior after all, they would have to be equals in some way.
Which leads me to a bit of a sub theory for this: that Morty has the potential to be almost as smart as Rick but due to either an actual mental disability, as has been suggested, or just because his family has crushed the potential out of him (for the most part) paired with his anxiety issues, he hasn't been able to really show it. I think Morty is just so used to being ignored or forgotten in his family before Rick showed up and after Rick showed up being kind of the scapegoat slash sidekick (depending on what family member we're talking about) that even he doesn't know that he has the potential. everyone knows the American school system is not exactly geared toward helping people realize if they're actually intelligent or not and we've all seen the kind of teachers that are at Morty's School, it's not exactly a shining example of Academia. It's a bit of a stretch but there is even a few things in the show and in the artwork that suggests this could be possible. season 3's artwork shows a Morty dressed as Rick and in one of the background scenes (I honestly don't remember which one, some point when they're in the Citadel) we see what is pretty obviously a combination of a Rick and a Morty.
But back to the original point, I don't think that the Morty waves are necessarily idiot waves as has been suggested. I think that Morty just happens to be kind of dumb and also happened to have complimentary / opposing brainwaves to Rick. it works out making it so Rick can be the dominant one in the relationship (not necessarily in a slashy way) the way he is obviously used to. I think another bit of evidence for this theory is that doofus Rick was assigned a Morty. it seems fairly obvious that that Rick isn't as smart as the others, otherwise they wouldn't treat him quite as badly (or he would be able to shut them up). Why would a less intelligent Rick need a Morty if it was the ratio between intelligence and stupidity that made them cancel each other out? It makes it all the more likely that it's just a convenient compatibility between their brain waves, no matter how intelligent or stupid either Rick or Morty is.