Hm... I'm both a mecha and a Gundam fan, and I know Tomino and his shows... aahh, good old Kill em all Tomino. Now, first off, let me say that this guy is definitely a person who deserves a ton of respect for his achievements, regardless of whether or not you are a fan of the genre.
That said, this isn't the first time he's commented on something in the entertainment industry that is... questionable. This guy is one difficult old geezer for sure.
It's pretty rich that he's taking a dig at Isayama working through childhood bullying in his work, considering he himself didn't earn himself that nickname of his because he was all about the butterflies or keeping his private life out of his works.
The story goes, when he was killing everyone in his shows left and right like there's no tomorrow (introducing characters just to kill em off? He's the one who came up with that, more or less, and made it into some kind of sport), he was working through his own problems in the form of several years of (pretty severe) depression, so not sure what his deal is with calling out Isayama like that. Honestly find that extremely baffling.
But this isn't the first time he has a dig at a fellow creator in the industry... he didn't exactly support his fellow creators of Gundam series he wasn't involved in either.... the reports on how badly he acted vary, but it's definitely not particularly respectful. He also called out the video game industry in a pretty nasty and uncalled for way- apparently it has no productive use (yeah, because being in the business of creating TV shows to while away the time is so productive. Pot calling the kettle black much)... so this is nothing new.
Comparing SNK to One Piece also seems decidedly silly, considering it's a totally different type of series. At least pick Bleach, if you want to compare it to a big shonen series. One Piece is not everyone's cup of tea. SNK isn't everyone's cup of tea. Both are successful. Why? Because luckily, people have very diverse taste.
While the guy has and always will have my respect, I despise this type of thing, it just isn't polite to attack a fellow creator like that. If you have fair criticism, that's fine. Taking issue with the artwork, even though it is something Isayama is well aware of and visibly working at, fair enough. Taking a dig like this by passive-aggressively insulting a young talent for the sake of it? Rude, but that's just the kind of person he is, I guess... he hates everything and everyone, so I'd me more shocked if he'd liked it.