Is practicing boxing and participating in its championships considered a minor sin or a trivial transgression (lamam)?

Normal boxing is forbidden because it inflicts harm on others and involves striking the face. If it is free from these elements, then there is no objection to it. Your participation in the permissible forms of it is fine. As for the forbidden forms, we fear that what they involve, such as striking others without a legitimate Islamic reason, may be among the major sins. Persistence in minor sins turns them into major sins. It is not permissible to be complacent about a sin on the pretext that it is not among the major sins. It is not permissible for a Muslim to be complacent about a sin on the pretext that it is not among the major sins.