I think it's because it's so easy to use, and sometimes it's lack of stuff it can do.
I know a few modding people who hate it because they feel like it's a program for cheaters. People who don't really make mods, or don't want to spent the time to actually code one.

Not too sure though. I love using it : D

The reason I am asking this is because I am setting up a 1.12.2 weapons mod(something simple) for my lucraft addon. basically, it will add in whatever I need that the addon itself cant. Or some other things/ideas I have along the way.

And not flat in 2d item, 3d models.

It's mostly because of it's lack of functionality, in my opinion. Many people open the app and without looking at tutorials make really standard mods that aren't all that creative.

That doesn't mean all mods made with MCreator are overdone and unoriginal concepts though, I've seen a lot of decent mods come out of it!
From personal experience, it's a great place to at least start. I have like, 0 knowledge in coding and my job doesn't really allow me to just sit down and learn too often. MCreator has actually helped a bit in understanding what bits of code to what, and how it all fits together into the whole modification. My first mod made on their was The Crystallia, which I've been tweaking for around a year now and have gotten a lot of positive feedback for!

There's other mods such as "Ruby/Emerald Tool" Mods or mods that just add stuff that thousands of other mods already do better that put the bad stigma on MCreator for just being a cheap way to churn out a large quantity of under average or on-par mods.
From what I've heard MCreator also uses bad coding, though I don't really know what's bad about it.

I made a mod around 4 years ago and it was a huge hit and lots of people loved it, I've never received hate of any kind for that mod... I have remade that mod like 2 weeks ago tho, and even with MCreator, it took me more than a few hours to complete.

Ofc what impresses people is the work put into a mod. And developing a mod takes a whole lot of it. And when you see all of your hard programming work being automated by some program, it might make you feel upset.

But look, I'm a developer, a web developer, I don't do Java anymore. I have ideas for a mod and MCreator lets me do that easily. I can focus on content making rather than having to mess with code.

MCreator has evolved a lot since back then, it's now a fully capable IDE for modding, in case you really want to make your mod do more than what MCreator already allows you to do, and code your own way.

As long as you make content that people genuinely enjoy, it's alright to use tools to make that happen!

One of the reasons surely is the generated code. It is way too messy - especially after adding the procedures. It makes the code not just harder to read but also slower (and possibly more buggy?).

Another reason may be the fact, that most of the mod (at least in last year or so) are limited to a few limited types that are used over and over with little to no innovation. Sure it can be cool to see nether-like dimensions made out of mycelium or a mod with 1500+ random items/blocks, but it is cool for like 5 minutes as they usually have almost no real depth (and again - almost nothing new with exceptions of names/textures). Of course, not all MCreator made mods are like this. The MCreator mod that I have always admired the most is the one that added coins, wallets, shops with NBT data (I do not remember its name though...). Another cool example is the MineScript mod that allowed you to run commands from a file on your drive. That being said, these two are not enough to make it up for the thousands of op weapon/random nether clone mods.

If I had to say why I personally do not like MCreator that much anymore, it is mostly the direction that the development went.

I know back in the day, MCreator was disliked because it rewarded less effort, rather than more. So for example, if a mod creator had created this amazing mod by himself, people are impressed because it took a lot of time and effort in order to make it. But then you see an amazing mod made in like 10 minutes on MCreator because of how much hand holding is in the program, then you'd probably be mad or at least you would be if you were the guy who put all the effort in.

Haters gonna hate.