I think they are similar because in happy day in hell, Charlie is talking about how today might be a good day and she describes how everything in hell is amazing and warm. Mountain Town is basically the same because Stan at the start says how great his town is and talks about how amazing the day is going, so I would say both of those songs kind of match!!
Also the melody is sorta the same
Songs:
Happy Day in Hell - Hazbin Hotel
Mountain Town - South Park: Bigger, Longer & Uncut
SONGS ARE NOT OWNED BY ME!!
Both tv show and movie can be watched on prime video