Although there is already a best answer, I am going to add my thoughts because I feel there is something else to be said: just because a Pokemon is more common or generally better than another Pokemon, doesn't mean that you should just slap it on your team without a second thought.
Yes, Tapu Fini is a very good variance control mon and more popular than Amoonguss on the ladder currently, and yes, misty terrain helps combat dragon moves, but there are no dragon types or any that threaten your team, and yes, it does prevent status conditions, but you only have two physical attackers that care about burn, one of which isn't grounded and isn't protected by misty terrain, so in this case, Amoonguss would actually do a better job protecting your team from status, since it can just redirect it.
Additionally, and this is the big point, your team has a Trick Room mode employing Dusclops. Dusclops can be taunted by things like Incineroar and Whimsicott, OHKOd by threats such as Calyrex-S, Yveltal, and Moltres, or Reverse TR'd by Porygon2. Tapu Fini can't do anything about this, and a lot of threats can KO Dusclops through Regieleki's screens, fake out eleki with its partner, or ignore them completely (Urshifu). Amoonguss solves all these problems: it redirects deadly attacks and taunts (Helping hand Life orb Astral Barrage from Calyrex ignores redirection and still OHKOs, but this can be easily worked around by setting screens before bringing Dusclops in or baiting out a Dynamax) and can spore Porygon2. Also, Amoonguss makes your Trick Room mode a lot more threatening, since sporing opponents before they can move exerts a LOT of pressure.
So, the bottom line is: currently, yes, you could call Tapu Fini "better" than Amoonguss because it is more popular on ladder and has more attributes on paper than the mushroom, but that doesn't mean it's the best fit for your team. In this case, there was a reason why the teambuilder chose to use Amoonguss, and it's that its niche and/or role will most definitely help your team win more games.
Finally, one small point I can make is that out of the top 21 teams in the first big Series 8 Tournament, there are 11 Amoonguss and only 3 Tapu Fini, which points to the fact that Amoonguss might actually be better in high level VGC.
