(To start, by mud slap, you mean mud shot, right? Garchomp learns mud shot and not mud slap)
Dragon Tail paired with Outrage is the premier combo for a strong Dragon type in Gyms and Raids. Garchomp is a good dragon attacker, but there’s a plethora of dragons, such as Salamence and Dragonite. Garchomp can definitely be a dragon attacker, but it’s got lots of competition on the Raids and Gyms side. Mud Shot is the better option in PVP for quicker energy gain, allowing it to unleash devastating moves such as Outrage and Earthquake more easily.
Garchomp is one of the best grounds there is. The community day made it far easier to obtain candies, and also gave it Earth Power. Ground hits 5 types super effectively, tied for the No.1 spot with Fighting. Dragon also gives it extra resistances, giving it more “bulk” against Electric typed enemies as such, which makes up for it’s mediocre bulk. On the PVP side, it’s preferred for it’s greater energy gain. Earth Power is a good charge move for PVP as well as PVE, with high power and 2 bars for PVE and a relatively strong and relatively fast charging move for PVP.
In the end, I’d say Mud Shot is more valuable in more situations than Dragon Tail, while Dragon Tail can be useful at times.