If I play Icathian Rain and my opponent retreats two units leaving only Pouty Poro, am I forced to target the Poro 6 times and pay 6x Deflect costs?

You must target Pouty Poro 6 times (since it's the only legal target), but you are not forced to pay the Deflect cost. You can decline to pay for each triggered Deflect ability, and if you do, those damage instances will not resolve.