Creators will point out if it is not allowed or have a fan-merch policy, but generally as long as they are not mass-produced and the art is not stolen, it is acceptable.
Here's the fan-merch policy for Undertale as an example : https://undertale.tumblr.com/post/139726155020/changing-policy-on-fan-merch
I try to follow the rules as best as I can
Ahhh thanks so much!! It means a lot, really, really appreciate it!
Aw man these are great!! Thank you so much!!