He can't stall games for 50+ min after the new Baron was added and his ult. no longer instantly waveclears since its bonus damage vs minions was removed. Basically, the biggest reason to pick Ziggs in the past is gone now (good riddance) and the tower execute doesn't make him strong enough to be a worthwhile pick.
His poke is potentially decent but it's too easy to dodge for it to be that scary. In lane, he has basically zero kill pressure vs competent mids and thus fairly low snowball potential while being pretty free vs some popular mids like Ahri. His waveclear is fine but not even best in class. Viktor can do it better while having way more burst and CC. His escape is nice but its unreliability makes it no better than just having a hard CC spell.
I don't think Ziggs is ever the best choice in a teamcomp atm and the main reason is that he doesn't snowball as well as other mids while also not having a dominating late game. He needs slightly better scaling. His CC is pretty low so his damage needs to make a great impact.