Probably.

But Columbo justified it (with a real specific reference) in his talk to the class in Columbo Goes to College.

How about when Columbo jimmies open Leslie
Williams' locker at the airport in "Ransom
for a Dead Man"? Any evidence he would have
found there would have been inadmissable in court.
Generally we don't see Columbo blatantly violate
the rights of his suspects, even if he maneuvers
them into incriminating themselves.

good points....also, how about when he tells the young policeman to go ahead and search general hollister's yacht, saying that he will get a warrant later if he needs it.

Interesting point about the gum in "Agenda For Murder." Columbo's real evidence is the piece of cheese, which apparently coupled with the other circumstantial evidence enables him to get a court order for a mold of Oscar Finch's teeth.

It is interesting that despite the search for the gum being illegal, it gives Columbo a head start to the final clue.