hey @Quyen-N The legality of Dapp lotteries is complex and has no one-size-fits-all answers as the law governing gambling varies from jurisdiction to jurisdiction However some general principles that can be applied according to me will be :
Depends on jurisdiction
May be considered gambling
Requires licensing in many jurisdictions
Should not target minors
Should be transparent and fair

And most importantly the risks can't be ignored like shutdown by regulators , Hacking, and money laundering.
Hope this Helps