Why is the nose cone of Russian fighters pointing slightly down?
The nose cone position depends upon the mission profile of the aircraft. Su-30 is a multirole fighter for all-weather, air-to-air and air-to-surface deep interdiction missions, since its air-surface with deep interdiction it needs to have a view of the ground while targeting ground troops/equipments/artillery. This nose down config helps it to have that view. F-18/F-16/F-15 are air superiority aircrafts. Yes same cannot be said for F-18 but then F-18 has many variants, F-18D which has air-ground support is slightly elevated pilot view for better ground view. Same can be seen in Eurofighter Typhoon and A-10 Thunderbolt.
While Su-15, Su-24 and Mig-27 where interceptors and bombers, they have straight nose just like their companions from the west.