@Keith Mowz  
Who? J/k, I do agree with you in fact. My point was actually that it makes more sense to compare with characters coming from the same season rather than with characters who were introduced less than six months ago – three weeks in Gladmane’s case – (and even then, Mrs. Shy got nearly 200 art pieces to her name in less than four months and I’m confident that she’ll get many more in no time).  
Silver Shill is a better example in this regard, as he’s also a character from season 4. But like you said, he was a rather forgettable character, despite his role as a key giver, and ultimately failed to make the cut as a fan favorite (unlike Coco Pommel or Cheese Sandwich before him), considering that his tag only got 122 hits, screencaps included.
 
Seabreeze was a bit more solid as a character – even if he’s nothing close to Maud Pie in term of popularity* – and therefore he appears in almost 400 pictures (300 if you discount screencaps).
 
So at the end of the day, it comes as no surprise that any sufficiently prominent character would get as much, if not more art than the comparatively more obscure fan creations.
 

@Ereiam  
I mean yeah but there have been characters like that in pretty much every season. They’re just hard to remember because no one ever draws them, eg, Silver Shill.

@Keith Mowz  
Those three characters only appeared this season though, so time is also a factor (even though I feel that Fluttershy’s mom is the only one who could overtake Tatzljack in the long run).
 
On the other hand, I’m also pretty sure that the chimera from “Somepony to Watch Over Me” (the episode where the edited screencap in which Tatzljack was created came from) doesn’t have as much art as Tatzljack.  
For that matter, even the tatzlwurm itself doesn’t have as much art as its apple pony derivative.

@Arifproject  
There’s plenty of canon ponies that don’t get that much art. Gladmane and Dragon Lord Torch have less than 100. Even Mrs. Shy has less than 200.

@Arifproject  
You call 200+ “only a few”? For a fan creation, I’d say it’s pretty impressive already.  
To put things in perspective, plane ponies have been around for longer than tatzlponies, yet they don’t have much more art comparatively (here on Derpibooru at least: 652 in 4 1/4 years vs 344 in 2 1/2 years for tatzlponies).

I’m surprised that there’s only a few Tatzljack art currently (below 250) :3

The original tatzl!