Just good etiquette - you're well within your rights, but not all artists are comfortable with having their work copied, even non-commercially. So,e ven though you can do it legally, it's still best to do the polite thing and ask for permission.
Hi! I'm quite sure permission isn't nessesary in this case. That's non-commercial usage, I don't selling this art or T-shirt with art on it, it's just a pixel art version. And yes, there is my sign on it, but also I mentioned author everythere I post it. So, I don't see any issue here.
Do you have the artist's permission? This is a very close copy of the original rather than something merely referenced. They'd probably be flattered, but it's best to ask in such cases.
5 meeseeks out of 5!