Poison has always been strong though, it’s just that cobra especially has been pushing to make it more available without a bunch of limited legends. I guarantee if the mortar monsters were all in the normal eggs we’d be complaining about the mortar meta instead.
As far as countering poison goes honestly the easiest way is to just focus on value. Most poison monsters are generally reliable but low value sweepers compared to newer monsters, there are a few notable exceptions (bane and terror) but the majority are single target with pretty high tu, so you can beat them by using monsters that simply generate high raw value (revenarchion, delugazar, etc).
At least that’s my experience of them anyway.