The phrase "mere shadow" means something that is very weak or insignificant, like a faint reflection or a small part of something bigger. Full definition
The four main characters here aren't mere shadows of the ones from the original film and its sequel. (markreviewsmovies.com)
By the time 12 months pass, they are mere shadows of their former selves. (soupaddict.com)
The death of Christ, therefore, is the point at which history becomes fully real, exhibiting no longer mere shadows, but «the very image of realities (x. i). (religion-online.org)