Yes there are problems with this strategy.
- Tailwind does not need to be Baton Passed, you can use it and switch normally and whatever you switch to gets the speed boost.
- Tailwind would wear out before you got all the Speed boosts to Mawile.
- Using Smeagle and Ninjask on the same team is incredibly predictable and easily played around. A person will see it and bring out their taunter, phaser or other things that counter Baton Pass.
- Agility raises Speed by +2, so you don't need to use it six times, just three (assuming that maxing out Speed is your strategy).
- Mawile does NOT need that many Speed boosts. A simple +2 is enough to outspeed most unboosted Pokemon, and even that is too much attention to speed. Besides, I personally don't agree Mawile should stay in against things like Lando anyway. It will survive and beat you anyway, specifically is it's Therian form and has Intimidated you.
- Mawile already has Sucker Punch to move first and revenge kill things, so Speed is kinda unnecessary.
Honestly doing that is WAY too much effort put into supporting Mawile. Having two weak support Pokemon is not ideal, especially when their purpose is to make Mawile outspeed things that will beat it even if it moves first. If Mawile is going to be outsped an killed, just switch, don't waste a third of your team on support Pokemon.
Also this is assuming that you're doing Wifi battles, if you're on Showdown don't even think about doing this strategy. Ubers is way too much for Ninjask and Smeargle to handle.