Loads of strikers better his ratio at international level so I’m not sure what you’re getting at. Berbatov at 0.61 has a better ratio, Javier Hernandez is slightly worse at 0.47 to Lukaku’s 0.55, playing for inferior sides too.
Since his international scoring record really took off in 2016 he’s scored 23 of his 32 goals against Panama, Tunisia, Gibraltar, Cyprus, ROI, Finland, Iceland, Scotland, Bosnia & Herzegovina, Estonia, Saudi Arabia and Costa Rica.
You’re talking teams that range from non-league to Championship level which is why it means little to me. United rarely play sides of such low quality and when they do players like Lukaku are often rested.