There's nothing wrong with it if you have fun; I guess I'm just trying to understand your code of trophy hunting. To me, playing bot mode is about the same as boosting. You might not get it done as fast, but there's no challenge to it. You would just be going through the motions same as boosting but against sub-par A.I.
Bot mode has typically always been for when you want to dick around and learn the map or flesh out strategies with teammates. Other than that, it's reserved for people that struggle online or when the community dies.
No, there's no rule that says you have to play that way but that's typically what they're for. The campaign isn't anything special and tends to drag at points so if you don't think you would enjoy the multiplayer then I would pass.
Some of my trophy "rules:"
No glitches, no cheats, no boosting.
A glitch is anything that requires specific, technical steps or is obviously outside of the game design (ie: any infinite health or money glitch).
Cheesing is okay. Workarounds are okay. If there's a spot in a map where I can't get hit and all I have to do to get there is walk to it, that's fair game.
If I can get a multiplayer trophy without actual people but I'm not gaming the system by doing it (ie: boosting), it's fair game.
Setting up games to favor earning trophies is not considered boosting (ie: I need pistol kills so I create a room that only allows pistols to be used).
If I'm letting someone win or they're letting me win, I consider it boosting unless the trophy is just for completing matches.
I don't quit to dashboard or overwrite saves on anything.
There are many grey area situations that are up to my discretion. Things borderline on glitch or not and I'll decide if it is or isn't.
I can watch videos except for puzzle games, although if I'm completely stuck, I'll look up the next step or two. Basically, until I'm stuck and have completely given up, I'll look up the answer to move forward.
I'm sure I have other rules but that's what I have off the top of my head.