I think it's a valid warning. Comic books are often open to casual reading, but some have such massive backstory that they really aren't, even if they are ostensibly episodic. Graphic novels, in the literal sense, are books told in visual imagery, so like other novels they don't lend themselves to picking up just one chapter. It's good to let potential readers know what they're getting into.

My audience and I have built up a generally familiar set of warnings. Some of them are things I've specifically been asked to indicate like "Asking for help and getting it" or "This includes graphic descriptions of delicious food you may not be able to find or make." Once in a while, someone bitches about it, but the approach fits my audience and I'm not giving it up. Same with my footnotes. I have readers who are from different cultures, the whole QUILTBAG, had a deprived childhood, etc. and they need those references. About 19 times out of 20, people appreciate it; as for the other one, it's not addressed to you, just skip it, but don't diss what other people really do need.