Ok... im not talking about where one party just cant get good grades in school or one party is really slow. I mean low IQ wise, not just lazy or a bad student or a delinquent. Someone who isnt necessarily mentally retarded, but not the brightest person at figuring stuff normal people can easily figure out. I dont know if such a manga exists, but I would like to see it...
If what you're asking for is "someone" being naturally stupid then I think 90% of the shoujo heroines are like that. Most shoujo manga i've read have always had heroines doing bad in school (not because of laziness or just being a bad student). I guess you can just try picking a few shoujo manga though if thats not you're looking for then forget this reply was ever there lol