IF (note the capitals) the person was making inferences or blatantly claiming that this was somehow they're original idea and work, that is, they were not deriving their work from another, then sure "problem".
However, this doesn't fit the definition of plagiarism. It also doesn't fit the definition of 'ripping off'. They're not making money through a failure to attribute.
From reading the OP, I gathered:
1. Bill Watterson created a cartoon.
2. Bill created the characters
3. The OP really likes his work
4. The OP (or another person) created this tribute work
Unless I'm missing something, it appears perfectly clear that the OP and the creator of the art work (if one and the same) has clearly identified where they got the original work from.