If I have 6 cards in my hand and use Mimikyu's "Impersonation" attack to discard Erika's Hospitality from my hand, can I use the effect of Erika's Hospitality?
Since you have 5 cards in your hand after discarding the Erika's Hospitality you can use its effect.
Source: TPCi Rules Team (2023-04-06)

Relevant cards

Note: All applicable cards may not be listed below.
SM12_96
SM12_96
SM9_140
SM9_140