You can't use a thunderstorm because whether that was create by a divine mind or not is still in question.
Although your premise that a mind wasn't necessary to create the mud art seems reasonable, that's venturing into ID debate territory, this is just to establish the default, if not easily refuted position for the issue.
So, you see, the argument has now become vacuous. What you're saying is this:
1) We split the universe into two mutually exclusive objects:
a) Man made objects
b) "Natural" objects (IE: non-man made)
2) All man made objects were created using mind or intelligence.
3) Therefore "natural" objects probably were, too. (I say "probably" since you're just trying to establish a default position)
But this just playing with definitions. Of course all of set a) has been created by an intelligence, that comes from its definition of being man made.
I could make the same argument:
1) We split the universe into two mutually exclusive objects:
a) Pink objects
b) Non-pink objects
2) All of set "a" is light red.
3) Therefore all non-pink objects probably are light red, too.
Well OF COURSE all pink objects are light red. They mean the same thing. Step 2 can be thrown out. All you've essentially done is make a definition and claimed the conclusion.