Oliver Marmol has done quite well at utilizing the roster (and coaching staff) he was given.
Contrary to what some fans believe, the manager is not responsible for forming either the 26-man roster or the 40-man roster on a baseball team. Those decisions fall on the head of baseball operations. In the case of the Cardinals, that would be John Mozeliak's role.
Now, it's possible the manager has input on who is called up and who gets sent down over the course of the season, but the final call is the general manager's to make. That means the field manager must work with what he is given. In the case of the Cardinals this year, Oliver Marmol's 26-man roster was below average at multiple times throughout the year.
At the start of the year, his bench included Ivan Herrera, Matt Carpenter, Michael Siani, and Brandon Crawford. That's not an overly inspiring bunch, and it's a pretty one-dimensional collection of role players. Three of those players hit left-handed, one is only a catcher, one had virtually no defensive viability anymore, and another was an aging backup shortstop. Marmol didn't have many options when it came to late-game maneuvers from the offensive side of the game.
Oliver Marmol also wasn't dealt the best hand when it came to pitching decisions. Prior to Andre Pallante taking the final rotation spot and running with it, Marmol had just four true starters: Sonny Gray, Miles Mikolas, Lance Lynn, and Kyle Gibson. Again, that's not a high-ceiling rotation. John Mozeliak was hesitant to promote a Triple-A pitcher to fill the final spot, and Marmol had to weave bullpen games in while managing the rest of starters at the time.
Something else beyond Marmol's control would be the staffing and technology that the organization provides. Be it the coaches on the field, the lack of support staff, or the dated technology the organization uses, very little blame can be placed on Marmol's shoulders.
Oliver Marmol has done the best he could with what he was given. It was his idea to stick with Andre Pallante as a starter and Michael Siani in the outfield. His usage of the bullpen, however maligned it is online, has been very strong (the relievers are eighth in win probability added). Marmol hasn't been given the most ideal lineup all year, but he's used it in the best ways he could by and large.