Haha! "Oi eats me spinach!" I think it may have been done to make him look more "basic" as a charracter. It actually sounds more like rural English than American to me. Interesting!

Popeye is a cartoon character who eats spinach and becomes very strong when he eats it. Looking at the lyrics you will notice they include speech marks "". These show that it a person's speech we are hearing. As such this allows you to change words and misspell them so that they sound more like how to character says them. If you search for Popeye on youtube and listen to how he speaks then you can pick up the differences to Queen's English or other english dialects. It does indeed show that he may be uneducated - but more it shows his character. Finich would be finish cause = because It is similar to the phrase - The water in Majorca don't taste like what it oughta. Which could be called slang. And someone with 'education' would say - The water in Majorca does not taste how it should.