Are Smogon's tier lists remotely accurate or balanced for competitive Pokemon?
It seems like they have no idea what they were doing with some of their placements. Almost as if the tier list was made by people who have only played solo battles; and never used non-attacking supportive moves. I guess it makes sense, since it was made by a large community, that it would be skewed by the opinions of bad players and people who aren't used to advanced strategies, but competent players can look at it and see it's mistakes right away.
I understand that it's focused around single battles, but they still have super silly tier placements even in that game mode. Have the people who made that tier list ever even used a defensively built Pokemon?
Smogon tiers are based on usage. If a Pokemon drops below 3.406% usage in a tier, it drops to the tier below it. When playing in a given tier, Pokemon from higher tiers are not allowed, but Pokemon from lower tiers are fair game. At the end of each month, usage is calculated, and tiers are readjusted accordingly. If a Pokemon is deemed unhealthy for a certain tier, they are placed in separate, non-usage based tiers. Pokemon banned from OU are placed in Ubers, Pokemon banned from UU are placed in BL, Pokemon banned from RU and placed in BL2, and so on. The idea is to allow as many Pokemon as possible to thrive in. I'm not sure what you mean by there not being any top tier defensive Pokemon, as the likes of Chansey, Ferrothorn, and Toxapex are all OU.
You might also be thinking of viability rankings. Viability rankings are separate from tiers, as while tiers are based strictly on usage, viability rankings are a measure of how well a given Pokemon performs in a given tier. Not all Pokemon in a given tier perform equally as well, and sometimes a Pokemon from a lower tier will have a small but effective niche that warrants being ranked, even if it isn't used enough to be placed in that tier. For example, Gastrodon has a B viability rank in the OU tier, due to being able to take hits from things like Greninja and Tapu Koko. However, it doesn't have enough usage to be OU, and since the things it walls aren't allowed in UU, RU, or NU, it dropped all the way to PU. Due to the subjective nature of viability rankings, they are decided right here in the forums (for reference,
here is the viability ranking thread for OU, though all tiers have one). Anyone can nominate a Pokemon is rise or drop in viability, or give their thoughts on an other's nomination. Every week or so, a team of knowledgeable users vote on the nominations. Once again, I don't know where this "no defense" mentality of yours is coming from, as Chansey is A-, Ferrothorn is A+, and Toxapex is freaking S rank.