Mcreator response/thoughts (speculation not fact):
MCreator is great for those that are getting used to modding or want an interface/templates to use just like say other Visual Programming means like Scratch (prefer to image 1, that uses coding blocks to make it clear what actions do what, similar to when Minecraft had it's teach kids programming code block thing).
I've used yes both MCreator before and done proper mod making (even if a WIP still but I still understand it a fair amount so far).
I assume people hate it because it's not making 'real' mods because they use an interface or aren't really coding, or it's the quality of it looking like a template or the assets/textures aren't that great and so on.
I mean yes many are tools, armour, food/drink (with status/potion effects sometimes) or otherwise mods, and you don't get many with dimensions that look less like a nether/end template or have blocks with GUIs but they are still pretty good from those I've used. The only problem I have with them is they can be hard to learn/are less clear without a question from the author or a wiki (for the much bigger mods with blocks to interact with my mean).
Mods I hate (to keep it on topic):
I do hate tool mods when I see hundreds of them (but over time I just pick one for an in-between iron and diamond tier and am ok with it) or the mobgriefing mods (compared to using the command) but I get they are for people that don't want to use commands (or can't if on a server) or want it for specific mobs compared to the scale I'm after.