I can only assume Skrelp was decreased because by then there were now plenty of Dragon-types that getting the Dragon-type medal was no longer such a major accomplishment. When Swablu was released there was only the Dratini family, Horsea family, and then also added the Flygon and Bagon family (I'm assuming this, feel free to correct me if I'm wrong). There were so few Dragon-types they likely felt it would cheapen the medal if Swablu could be used to farm Dragon-types. But by the time of Gen VI? They would have added the Gabite family, Axew family, Druddigon, and Deino family. They were now plenty of Dragon-types, in addition to Noibat and Goomy also being added (or were to be added). It just wasn't worth being so gatekeepy anymore. Noibat being 400 is an oddity, but already being a Dragon-type I wonder it's another issue why it's so high, like maybe they were worried Boomburst was going to be so powerful?
In terms of other Dragons released when Swablu was, Rayquaza was introduced on the very same day as one of the absolute easiest raid bosses of the time, I remember people saying that Rayquaza was a more efficient way of farming the dragon medal than evolving Swablu, despite requiring a raid pass per encounter. Bagon was also released the same day, and Trapinch a couple weeks earlier, the only dragons as of Gen 3 unavailable at the time were the Latis which came about two months later
The big thing, though, was that they hosted a Dratini community day just over 2 weeks after Swablu's release, which was easily the best way to get the medal up to gold and made Swablu farming nowhere close to even a consideration for people who were able to play
For Noibat, it's still pathetically weak as a Noibat, and I'm fairly certain boomburst being too good was not a consideration with its normal typing being a major hindrance by default. It's probably just to do with it being a weak pokemon that's a major challenge in the main series to raise to evolution.