In general both Mario and Luigi are easy characters to pick up and play, they were most likely designed like that, both character are usually well rounded with them not having to many weaknesses, while also not having to many defining strengths. The only thing really different between Mario and Luigi is Luigi has a stronger air game, but lacks in mobility and doesn't have many good approach options.
Now to address some "issues" you have with Luigi.
While true Luigi has a strong combo game after a down throw, so does many other characters such as Ness, Mario, and Lucas to name a few, it's not a rarity to find a character who has combos out of a throw, and as with many characters, with good DI Luigi shouldn't be able to hit you with multiple aerials after low to low-mid percents.
Luigi also isn't the only one with a fast natural air that can break combos, both Mario, and Ness (and probably and few others) have fast neutral airs that come out on the same frame as Luigi's neutral air, unlike Ness, Luigi's neutral air lasts pretty long and is not safe on shield, so if the Luigi always nairs and becomes predictable you can easly shield and then get a free grab.
As for Luigi's ability to gimp characters I am going to guess that you are referring to Luigi's cyclone gimp, other than many other characters having good gimping ability, to perform the cyclone gimp consistently you have to be able to do the jumpless cyclone which requires you to press the b button around 14 times pre second, which is a feat many Luigi players have trouble with, the only Luigi's I know of that can do the jumpless cyclone other than myself, are Mr.ConCon, and Boss (both of which are exceptional players helping advance the Luigi meta, and are not really a good players to say all Luigi mains can get these results, sense they are by far some of if not the best Smash 4 Luigi players there is).
Though many people see Luigi as top 3, and have trouble against Luigi, I personally feel that Luigi is top 15 at the highest, due to Luigi having some very hard MUs as well as having very exploitable weaknesses. Luigi has a very hard time against players that know how to play against Luigi, Luigi's lack of mobility as well as a lack of good approach options and very linear recovery, becomes very exploitable and gives Luigi a hard time. As the meta advances more people will start to exploit Luigi's weaknesses, making it harder for Luigi to get a grab thus making it harder for Luigi to rack up damage quickly, and then I feel Luigi will get a more appropriate tier ranking of top 15 and not top 3.
Pyr
well, I wouldn't consider myself great at smash 4 right now, but I would certainly consider Megafox great. I guarantee he's a much better player than you and he holds the same sentiments on Luigi in that he is a very cookie cutter character. So calling me out on fundamentals make no sense when one of the best smash 4 players in the country believes the same thing.
Ndzablou
I appreciate that you made a well thought out response to my immature post, but I still can't agree that he's no better than top 15. He has to be top 5 as he a pretty good camping game with fireballs, probably the best combo game, and a good recovery. While his recovery is gimpable, only a handful of viable characters can do anything about it, like Sheik and Ness.