Clown is weak, but only because chases are too strong for survivors
Clown, unlike other killers, can't do anything but chase. The reason Clown is weak is because chases take way too long. There's always another window or pallet, perk to extend the chase, or character movement to throw off the weapon tracking.
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Same could be relatively said about Doctor.
I've already covered his problem, but it mainly boils down to the fact that survivors have learnt how to counter his power effectively. That is it. His counter is harsh, and it's easy to use.
Doctor has much more than just chasing.
Yeah, he has information with it. Point was that they both have an AoE chase power that is countered in the exact same way, yet one character is much higher on the community's rankings than the other. Maybe it has to do with the fact that Shock Therapy also disables vaults and pallets, to which my answer is either bait them or do it earlier. The exact same thing that happens with Clown. Just with slightly less risk.
It's because Doctor has tracking and STALL on top of his chase potential.
Clown can chase but gets gen rushed easily.
Doctor has similar chase potential, but is much less vulnerable to gen rush.
Also, the tracking shouldn't be underestimated. Patrol times are a large timesink for a lot of Killers, clown included, but not for Doctor.
Shocking Survivors also cancels all actions in general, not just vaults, which has utility outside of a chase, especially since it applies to static blast.
I never said it was a perfect comparison, but yeah. The main issue still is that Clown's power is very easy to counter at base kit. And while the same may be said for killers like Doctor, or even the fact that in a chase you will inevitably be hit by the Clown's gas, it still doesn't change the fact that he'll have a hard time getting to you because you take the obvious counter lying around and he can't reach you.
You asked why clown is considered low tier trash and doctor isn't.
Doctor has a bit better of a chase power, tracking, stall, better addons and misc utility.
Clown's chase isn't even THAT bad. However, a decent chase power isn't great when Killers that have excellent chase powers are considered balanced (ex: Pyramid Head, Huntress ect).
Doctors viability is proof of this. Decent chase and nothing else = bad Killer. Decent chase and a ton of utility = good Killer. Amazing chase no utility = good Killer.