For my own research about a week or so ago, I made a table of hits per tags on FIMFiction and compared it hits per tags on Derpibooru. I found some interesting observations about which ponies were more likely to have images (art, screenshots, gifs, and memes) and which were more likely to have writing.
Note this is a completely unscientific analysis. I did my best to get accurate counts for each regarding the quirks of each tagging system (Mane 6 tag on FIMFiction implies individual tags for each member, for instance, while the Mane 6 tag on Derpibooru is used in conjunction with individual tags). Some more details at the bottom.
* Character with least tagged images (of tags that are shared between the two sites): Moody Root (18 hits)
* Character with most tagged images: Original Character (609,987 hits)
* Canon Character with the most tagged images: Twilight Sparkle (283,991)
* Character with no tags (that has a tag on FIMFiction): Chip Cutter (0 hits)
* Characters with the least tagged fics: Chip Cutter, Crackle, Grace Manewitz, Moody Root, Mr. Stripes, Mrs. Hoofington, Sassaflash (1 hit)
* Characters with no tagged fics (but still have tags for some reason): Clear Sky, Hayseed Turnip Truck (0 hits)
* Character with the most tagged fics: Twilight Sparkle (66,357 hits)
* Original character with the most tagged fics: Original Character (51,176 hits)
* Character with the lowest ratio of Derpibooru Hits/FIMFic Hits (gets much more stories!): Princess Celestia (89858/29510 = ~3)
* Character with the highest ratio of Derpibooru Hits/FIMFic Hits (gets much more art!): Sassaflash (1485/1 = 1485)
* Character with the biggest rank difference between Derpibooru hits and FIMFic hits (Favoring Derpibooru): Sea Swirl (Rank #161 on Derpibooru vs. Rank #286 on FIMFic, 125 place difference)
* Character with the biggest rank difference between Derpibooru hits and FIMFic hits (Favoring FIMFic): Clover the Clever (Rank #255 on Derpibooru vs. Rank #168 on FIMFic, 167 place difference)
* The order of Mane 6 tag rankings on Derpibooru is Twilight Sparkle, Rainbow Dash, Pinkie Pie, Fluttershy, Rarity, and Applejack. Their rank on FIMFiction is nearly identical exceptFluttershy and Pinkie Pie switch places! Fluttershy gets more images and Pinkie Pie gets more fics!
* Luna has more tagged Derpibooru images than Celestia. Celestia has more tagged FIMFics than Luna.
* The CMC have the tag rank in the same order on both sites (Scootaloo, Apple Bloom, and Sweetie Belle) as are the Pillars (Starswirl, Somnambula, Rockhoof, Meadowbrook, Flash Magnus, and Mistmane).
* I included a geometric mean column to average both sites. I used the geomean instead of the more traditional arithmetic mean because there’s a magnitude more Derpibooru images than FIMFics and a simple “add and divide” would weigh Derpibooru more.
* With the FIMFiction “group” tags, I added that total to the individual characters in that group. Does the FIMFic “Mane 6” tag include Spike? Evidence points to overall “no,” but it is kinda sad to see a major character fall so far behind the others.
* While each of the Equestria Girls Mane 6 get their own tags on FIMFiction, they do not on Derpibooru (barring AU Twilight, naturally). Granted, I could’ve tried using combinations of tags to calculate those, I didn't feel like showing the explicit stuff to get an exact count.
Regardless I feel like there's still enough interesting stuff to examine here. Also, feel free to copy and use for your own purposes, but please credit me!