He was criticized because he behaved like a mercenary, honestly, if he does his job and scores goals I don't understand the reason for the criticism. We all work for money, not for glory. In my opinion, whoever criticized him is a little inconsistent and stupid.
But I think the criticism came continuously after the incident at that time when Lukaku wanted to be made permanent by Inter Milan but was secretly close to Juventus so this behavior was considered unfair and considered an arrogant player even though it was normal behavior and did not violate the rule as long as just having closeness without having anything to hide wouldn't be a big problem.
But since that incident, Lukaku has always been doubted and some people even give him bad criticism after joining Roma.