I think this is where Plan9's comments come into play. If Matt is drawing his individual interpretation of a wacky character, and selling it, there is no issue. If he is mass producing topps images, that is a no no. He used the Andy Warhol - Campbell's soup example and I believe he is correct about this issue. Matt also doesn't use the wording wacky packages in his drawing. He only uses OS3, which is really a collector's abbreviation for the series. I don't even know if it is a common term outside the forum discussions.
Again, this starts to get into the nitty-gritty of Fair Use, etc. and is probably more complex than most people realize, and is probably only an issue if/when someone decides to enforce their rights.
But is this really his interpretation, or would most people deem it a direct copy of someone else's work?
Imagine, for example, someone selling their "interpretation" of Garfield, or Charlie Brown. If it is recognizably similar to the original it is no longer an interpretation. And "the 10% rule" afaik is not well understood (there is a lot of myth about it going around).
With the example of Andy Warhol, was he even violating any laws at the time? Perhaps many such laws didn't exist at that time and he was the first to do this kind of thing -- like the audio sampling that started in the late 80's and early 90's. It wasn't illegal and/or being enforced. Try to do that now and you'd be in trouble.
As for the commissioned sketch cards, my understanding is the artists are limited to doing only the characters from that particular series -- i.e. they can't explicitly use characters outside that series -- and they are doing the art on cards specifically meant for that series (return cards), which have Topps copyright information on them and are sanctioned by Topps.
Really the bottom line is whether or not people want to start the chain of events to enforce their rights (whether or not they actually exist) -- i.e. it comes down to money/effort.
Seems like Matt is doing unsanctioned knock-off's (possibly direct copies) that could affect the sketch card market. This seems very different from other examples cited.