No, it is certainly not legally safe to sell a shirt with such an image printed on it. There are two compatible facts to consider:
- Wikipedia is correct that the image is not covered by copyright in the United States.
The image is likely protected by trademark law, which Wikipedia also notes, directly under the public domain information:
This work includes material that may be protected as a trademark in some jurisdictions. If you want to use it, you have to ensure that you have the legal right to do so and that you do not infringe any trademark rights.
Copyright provides a monopoly on the reproduction of creative works, but copyright only applies to sufficiently creative works. In cases such as these, Wikipedia is expressing the opinion that the work in question is too simple or too utilitarian to meet the threshold of originality and/or creativity for copyright. For the U.S. specifically, consider 37 CFR 202.1 which enumerates some categories of work excluded from copyright protection:
The following are examples of works not subject to copyright and applications for registration of such works cannot be entertained:
(a) Words and short phrases such as names, titles, and slogans; familiar symbols or designs; mere variations of typographic ornamentation, lettering or coloring; mere listing of ingredients or contents;
(e) Typeface as typeface.
Category (a) excludes the word "Marvel" or "Nirvana" from copyright protection, while category (e) excludes the creative choices that went into the styling of the text. (Note that fonts -- the software that creates/renders a typeface on a page -- are copyrightable in the U.S., but a font's "output" of typefaced text is not similarly protected.)
Therefore, in the U.S., that "Marvel" image is likely not copyrightable, since it is just a single word with a particular stylized typeface (the use of which does not make it copyright-eligible either). This may be different from other jurisdictions, some which do recognize the use of the typeface as protectable element under copyright.
Trademark law is a completely distinct area of law that applies to the use of logos, phrases, and other elements to identify a vendor in commerce. The words "Toyota" and "Camry" are not eligible for copyright protection, but the government affords the Toyota Motor Corporation a monopoly on the use of those words to identify the source and model of a car. If you built your own car and tried to attached the name "Toyota Camry" to it without their permission, you would be liable for trademark infringement.
Similarly, logos that are too simple to qualify for copyright may still be covered by trademark law, if the company has registered them as trademarks or otherwise uses them distinctively in trade. This is likely the case here. If you put that rectangular Marvel logo on a shirt, you would be misrepresenting your product as being authorized by Marvel Entertainment and be liable for trademark infringement.