So basically, ugly Pokemon and Pokemon only designed to look defensive get to make use of high defenses, while cool Pokemon rot in the lower tiers as usual.
There are a couple of problems that I see with this. For one thing, how a Pokemon looks or whether or not it was "designed to look defensive" is completely subjective. I personally think all of those Pokemon I mentioned look cool except maybe Gastrodon, and Reuniclus and Conkeldurr don't exactly look "defensive" to me.
Second of all, I'm not sure what you mean by "rot" in the lower tiers. Pokemon in the lower tiers aren't rotting, their standing out. Let's take Hitmonlee in RU, for example. It makes a great offensive Fighting type in its respective tier, but it receives strict competition from Fighting types like Mienshao in UU and Terrakion in OU. If it weren't for lower tiers, it would receive little or no use at all. There's really nothing wrong with being in a lower tier. Being a lower tier Pokemon literally just means that you aren't used as much in the upper tiers, and it also means that you can be used in more tiers. If anything, it's a good thing, unless you choose to be offended by the term "Under/Rarely/Never Used".