Clown Has Been DBD's Worst Killer For 1 Year. He Needs Help.
Most players don't seem to understand how Clown's Power actually works. The short answer is, it doesn't. Basically it's placebo and most value gained from it is due to Survivors giving it more respect than it deserves, ruining a good loop just to avoid the gas. This has been known to a few since his release and the problems have never been fixed.
Remember in PTB when holding a Bottle slowed his move speed, similar to, but not as much as when Huntress holds an Axe? That was changed just before he released, but the base effect of his gas slow was nerfed.
What remains for Clown is a self-slow during his animation of throwing the bottle. It's fairly subtle, but any Clown main should've noticed this by now. You can feel the slowdown. It's been demonstrated that in beeline races, he runs at default Survivor speed if he spams bottles, illustrating the self-slow effect.
The net effect of this with no add-ons is usually zero in any chase, it varies slightly by how long the Surv remains in the cloud. This means you will reach a Survivor at the same time regardless of whether you use Bottles, however using Bottles has potential downside since if one of them misses due to e.g. phantom geometry collision and unreliable Bottle hitbox, Survivor gains net distance due to Clown's self-slow.
There are only 3 conditions in which using his Power benefits Clown rather than having no effect, or even slowing his chase:
1) The slow is enough to prevent Fast Vaults, turning them into Medium Vaults. In order to get any net value out of this however, the Surv must generally run through an entire Cloud's worth of gas slow, and then do a Medium rather than Fast vault, as simply the difference between Fast & Medium is still roughly net neutral if the Surv does not also experience enough Hindered duration.
2) Pallet/Jungle gym standoffs. Occasionally the Killer and Surv enter Pallet mindgames where the Surv's best chance is to run opposite of Killer, usually back and forth as Killer changes direction. In these cases, Clown's self-slow effect is minimal since the loop has come to a standstill and his Power can help him catch the Surv with precise play. In Jungle gyms this same mindgame can occur and the coughing can help alert Clown to which way the Surv has ran.
3) With add-ons (mainly Flask of Bleach) which improve the slow effect, resulting in something like a net +5% run speed for Clown, which isn't much but at least results in the Power doing something.
At the very least, his self-slow needs to be removed or he will basically remain a Trapper with no traps. A vanilla M1 Killer with a Power which is not worth using. And frankly to be decent at his job/niche, his Power would even then need a buff.
Ideally he'd get a more fun and engaging rework with another bottle type to cycle between, maybe a drinkable one which causes Exposed or gives him faster action speed, Pallet vaulting etc... he just needs help.