So, マンマ, or manma, is a loanword from Italian. It is the phonetic adaptation of the "mamma" using Japanese syllabaries. So, yes, at face value, "manma" can mean "mamma" (as in mamma mia!), but it seems it's not so commonly used in that sense in Japan. Mothers are usually called "mama", not "manma" by small kids, but "manma" does still have it's connections to parent and child.

More commonly, マンマ means food in baby talk. Obviously babies don't have the ability to sound out complex syllables, so for a lot of babies around the world, the first sound they can make to communicate is simply "ma". That can be interpreted as many things depending on the country and culture. In Japan, it's usually interpreted as baby talk for "food" and comes from 飯, read as meshi or manma.

The word also refers to breasts in the medical field and is derived from mammary, but I don't think that's relevant here...although Oda does love boobs and Gunko does have boobs!!!! 🤔

But jokes aside, if you think Oda would use a foreign word that isn't really used in the same way in Japanese in his wordplay, then sure! マンマ = mother. But I don't think anyone is wrong if they think it's a stretch either.

As for the "hate" part, that refers to the last part of the Manmayer name, イヤー, or "iya". The character 嫌 is read this way, more or less (the "ya" is elongated in the name Manmayer and isn't in the character). The dictionary definition from Jisho.org is;

disliking, not wanting, unwilling (to do), reluctant

unpleasant, disagreeable, nasty, horrible, disgusting, undesirable, unwelcome

no!, quit it!, stop!

"iya", and the character in this form, doesn't necessarily equate to "hate" to the extreme that the English does. It's not as outright or harsh, and we can see in the definitions above that the word "hate" does not appear.

When I first read the theory, my mind didn't immediately go to, "I hate my mother", but to something along the lines of, "mama, stop [ex: embarrassing me etc.]!" or "mama, I don't want to [ex: go to school today or eat my vegetables etc.]!". If you use "iya" in reference to your mother, it sounds to me more like, "My mom is so ugh (frustrating, complicated, difficult to deal with, etc.) so I don't want to interact with her." But you don't hate her.

To evoke the image of hate, 嫌い, which uses the same character but has a completely different reading, "kira-i", is much stronger and direct. You actually hate the person in the English sense of the word. Having said that, the claim of 嫌 meaning hate is not wrong because of that. The character as a stand-alone is used to mean hate by Japanese people regularly. There's just an extra tiny step to reach there linguistically.

So, does the theory hold weight? That's up to the individual. The theory has legs but it's not as straight forward as people on online would lead you to believe.

If after reading this, you're still on the theory train, there's a chance you are right. If you think you have to jump through too many hoops to land on, "I hate my mother", then there's a chance you are right. It really is 50:50.

What it really comes down to is, is this something Oda would do?

He is not a stranger to wordplay, but how obvious does he usually make it? I have no idea. I don't know much about Oda and I don't look out for that kind of thing when I read.

We probably won't ever know for sure unless he says it outright in an SBS or something. So, until then, the debate continues.