As far as I know Ugly Monster has a normal Loot System and not a Cooperative Loot System. According to Tibia Wiki all monsters with normal loot system are affected on Double loot event. Therefore the answer to your question is: Yes. Ugly monster is affected on Double loot event.
Source:
https://tibia.fandom.com/wiki/Double_Loot_Events
I'd like to confirm. Yes, Ugly monsters drop rate is affected during double loot event.
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