Currently doesn't feel mythic. Is it even better than Polymorphous Rush

Does this need so much blue in its mana cost?

Probably not.

The big difference between this and polymorphous rush is that you can potentially turn your opponent's creatures into canisters or other useless objects. That said, it does feel more rare...

If you have a 0/0 creature with counters on it, like Vigean Graftmage, this becomes "Destroy X target creatures that don't have counters or other external toughness boosts." It doesn't look like there are any of those in the set at all, let alone in blue, but putting one or two of them in for the synergy with this doesn't sound too bad.

It still doesn't make a compelling mythic even with the in-set synergy available, given Mirrorweave and its flat 4-mana cost.

Well, having tried moderately hard to use Mirrorweave, it's tricky. You need some kind of anthem or other boost because otherwise your creatures just all trade with anyone else's creatures. I mean, that's fine for a tokens deck, but it's a symmetry that needs work to break, where this comes pre-broken.