Toxicroak's relevance in the metagame has pretty much decreased once Drizzle got the boot (which was... last month? lol). Sure, it's niche in checking water types, but it's definitely not one's first choice for a fighting type. (Chesnaught arguably does that better)
As for usage stats, using January's (horribly skewed because of the bans, I know, but we don't have any others)... here's basically most of the 'offensive' knock off users you'll see around.
Code:
| 7 | Heracross | 11.74319% | 65869 | 10.886% | 52550 | 10.638% |
| 16 | Crawdaunt | 9.09813% | 51206 | 8.463% | 41606 | 8.422% |
| 31 | Mienshao | 6.26163% | 35295 | 5.833% | 30315 | 6.137% |
| 43 | Tornadus-Therian | 4.58933% | 23275 | 3.847% | 19645 | 3.977% |
| 49 | Machamp | 3.91324% | 23336 | 3.857% | 19347 | 3.916% |
| 78 | Krookodile | 2.52760% | 15860 | 2.621% | 13148 | 2.662% |
| 80 | Scrafty | 2.42783% | 16435 | 2.716% | 13374 | 2.707% |
| 91 | Escavalier | 1.82889% | 9610 | 1.588% | 8362 | 1.693% |
| 113 | Drapion | 1.11218% | 7722 | 1.276% | 6634 | 1.343% |
| 121 | Toxicroak | 0.95150% | 6669 | 1.102% | 5383 | 1.090% |
| 173 | Tornadus | 0.42510% | 2657 | 0.439% | 2183 | 0.442% |
| 200 | Hariyama | 0.28151% | 2157 | 0.356% | 1808 | 0.366% |
| 215 | Sawk | 0.24057% | 1758 | 0.291% | 1379 | 0.279% |
| 264 | Gurdurr | 0.11172% | 660 | 0.109% | 536 | 0.109% |
| 267 | Throh | 0.10636% | 805 | 0.133% | 671 | 0.136% |
Notice how Heracross, Mienshao and Machamp already has significantly more usage that Toxicroak. Heck, Machamp alone has more than twice the usage than Toxicroak/Hariyama/Sawk/Gurdurr/Throh combined. I hate to be that guy, but it is true that Toxicroak is pretty much a very rare sight in UU.
Toxicroak is never going to be as good as Bisharp in Knock Off/Sucker Punch abuse, as Toxicroak crucially loses out in 2 components: STAB, and Attack. Let's not mention how Toxicroak doesn't even run Knock Off that often (between STAB, Sucker Punch, Swords Dance and Substitute, it's really crowded for moveslots here).
Speaking of STAB: that's the one problem Fighting-types have: with the exception of Scrafty, none of them are getting STAB on Knock Off. This means that Knock Off is at the end of the day,
a fucking coverage move. Hell, it's even weaker than Earthquake. We already have Superpower/EQ Haxorus and Fire Blast/Earth Power Hydreigon for ages, and nobody's complaining. (well, except maybe the dudes who nominated Haxorus, but that's not really the point here)
Mienshao is already capable of U-turning on Mew and all the other Psychic-types around into a check. Knock Off just means Mienshao can do the job himself. (goddamnit i swear this is like the third time im mentioning this) I already mentioned that Machamp falls under 'disruptive' already with Dynamic Punch, Knock Off just further accentuates that. It's still pretty mediocre though because 90/80/85 bulk is pretty average and all it really has is DP. It's also susceptible to statuses and the like. It's not shit, but it's definitely not amazing either. Scrafty and Krookodile are the last of "threatening" Knock Off users (hint: STAB) that has some actual presence, but the former isn't strong without boosting and the latter has speed issues (though it's an acceptable upgrade from crunch).
Finally, Mienshao doesn't even 2HKO 252/252+ reuniclus with knock off without 2 high damage rolls.
Yeah fuck it, this guy isn't broken. Tornadus isn't even using it to sweep. It's a move meant to cripple the opponent. It cripples the opponent just like Will-o-wisp, Toxic, Thunder Wave and Trick. Just pack a remotely physically bulky Pokemon with some recovery, and you can use it as a Knock Off bait if your team somehow struggles with it.