A meme is an idea, behavior, or style that spreads by means of imitation from person to person within a culture and often carries symbolic meaning representing a particular phenomenon or theme
The comic strip format could be characterized as a memetic style of communication that has spread by imitation from person to person.
I'd say that it only "becomes a meme" once it's being shared around with small modifications, but when you see a comic for the first time then you might not know if that has happened already or not?
Originally, no, this wouldn't have qualified as a meme, but now it pretty much just means "any image that's shared online for the purpose of entertainment".
The definition is very vague now. And it definitely applies to any humorous images being spread using social media. And comics fall into that category.