"Too Underworld" means that the crime is perpetrated by criminals against other criminals rather than innocent citizens being victimized.
The primary case that Maigret is being asked to investigate instead is the vicious assault and murder of innocent citizens in Picardie.
Alternatively, the case that Maigret prefers appears to be underworld related although Maigret is not convinced.
In fact, it was made to look that way because the criminals know that the police will take it less seriously and only conduct a cursory investigation.
The victim's friend tells Maigret...
"The police don't take much interest if a gangster's found dead in the street. You don't sniff around that for long.
"So we dumped him like it was an underworld hit."
Maigret feels that this impression is false (clearly) but his superiors / senior colleague don't see it and have other priorities.
Of course, none of the investigators realise that the two investigations are connected until the end.