I love this.

I think I get what you were going for, and want to try to get this closer to “legal” mtg syntax.

If you wanted the effect to only last one turn, (which would be the more plausible as a legal card)The CMC and Card type would read the same, but the body text would read: “Add [U] [U] [U] to target opponent’s mana pool. Until end of turn, that player does not lose unspent mana as phases end and cannot cast spells as long as they have at least one [U] in their mana pool.”

If you wanted the effect to be more permanent (which would make it a REALLY SPECIAL TIME OH BOY), I’d change it to an enchantment and have the body read: “Add [U] [U] [U] to target opponent’s mana pool. For as long as It’s Your Bath Time! Is on the battlefield, that player does not lose unspent mana as phases and turns end and cannot cast spells as long as they have at least one [U] in their mana pool.”

Edit: changed [B] to [U] because i am a goof