Legal Question in Business Law in California

Unfortunately, a trademark, to be infringing, does not have to be identical, but only "confusingly similar" to a trademark owned by another party in relation to products or services which are identical or similar to the products or services you are making a commercial use of. Of course, this is not a clear standard and would be subject to the opinion of individual judges, jurors, experts, or your competitors!

Doing what you describe without obtaining a license to do so will get you sued as soon as the owners of the rights to the characters and logos find out. There are many unlicensed products on the market, and many get away with it for some time. But the owners of valuable marks and images have staffs of people who do nothing but look for unlicensed use of their intellectual property and shut it down, usually with expensive litigation.

The real value of your products will be that they are identified with the comic book characters. I doubt that a t-shirt made to look like the top of overalls would, by itself, be trademarkable. However, your primary marketing hook would be that it looks like Mario, and for that you would need a license from Nintendo (which has a licensing department that you should get in touch with). Sure, you could try to sell the t-shirts without making any connection to the comics, but without that, you'd lose the uniqueness of your idea and it wouldn't be worth as much.

Better to go the correct legal route and license the characters you want. It might not be as difficult or expensive as you think. (But understand that the company could just take the idea and do it without you.) If you fail, at least you wouldn't have spent all the money to make the shirts that you then have to turn over to someone else, along with a hefty penalty payment. (One of the penalties of copyright or trademark infringement is that you have to destroy or turn over all the infringing inventory without any compensation.)