I know Mothim might tear up some stuff in PU, but... it's still PU. The tier created because there were so many Pokémon that couldn't even work in the NU tier, generally considered the bottom of the barrel as it were. As stated by mertyville, Burmy has four evolutions, and neither of them seem able to escape the dregs of competitive Pokémon tiering any time soon. Sure, they can pull their weight against equally disadvantaged opponents, but neither of them will ever be seriously considered for top tiers of neither in-game lists nor competitive for the foreseeable future.
Okay then.
252+ SpA Choice Specs Mothim Air Slash vs. 0 HP / 0 SpD Tapu Bulu: 338-398 (120.2 - 141.6%) -- guaranteed OHKO
252+ SpA Choice Specs Mothim Air Slash vs. 0 HP / 4 SpD Volcarona: 308-366 (99 - 117.6%) -- 93.8% chance to OHKO
252+ SpA Choice Specs Mothim Air Slash vs. 0 HP / 4 SpD Keldeo: 354-416 (109.5 - 128.7%) -- guaranteed OHKO
252+ SpA Choice Specs Mothim Air Slash vs. 0 HP / 4 SpD Pinsir-Mega: 354-416 (130.6 - 153.5%) -- guaranteed OHKO
252+ SpA Choice Specs Mothim Bug Buzz vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 282-332 (69.8 - 82.1%) -- guaranteed 2HKO
252+ SpA Choice Specs Mothim Air Slash vs. 0 HP / 0 SpD Medicham-Mega: 372-438 (142.5 - 167.8%) -- guaranteed OHKO
252+ SpA Choice Specs Mothim Bug Buzz vs. 0 HP / 0 SpD Alakazam-Mega: 372-440 (148.2 - 175.2%) -- guaranteed OHKO
252+ SpA Choice Specs Mothim Air Slash vs. 248 HP / 216+ SpD Amoonguss: 278-330 (64.5 - 76.5%) -- guaranteed 2HKO after Black Sludge recovery
252+ SpA Choice Specs Mothim Bug Buzz vs. 56 HP / 0 SpD Serperior: 404-476 (132.4 - 156%) -- guaranteed OHKO
252+ SpA Choice Specs Mothim Bug Buzz vs. 252 HP / 0 SpD Reuniclus: 444-524 (104.7 - 123.5%) -- guaranteed OHKO
252+ SpA Choice Specs Mothim Air Slash vs. 12 HP / 0 SpD Hawlucha: 470-554 (156.6 - 184.6%) -- guaranteed OHKO (;-; Blizzard cries inside, but as long as you don't switch into it, and go double sub, Hawlucha will still be able to 0HKO with Acro. #TeamLucha)
252+ SpA Choice Specs Mothim Bug Buzz vs. 240 HP / 140 SpD Mew: 338-398 (84.2 - 99.2%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Mothim Bug Buzz vs. 0 HP / 0 SpD Greninja-Ash: 512-606 (179.6 - 212.6%) -- guaranteed OHKO
252+ SpA Choice Specs Mothim Air Slash vs. 112 HP / 0 SpD Heracross-Mega: 616-732 (187.2 - 222.4%) -- guaranteed OHKO
Some OU threats Mothim can chew through. I didn't include all standard sets, and I've also included sets that Mothim needs speed support (tailwind or webs, ideally webs) to 0HKO, but you'll have to suss out which those are.
Still somewhat niche, but it's not something to laugh at. See, Smogon tiers are based upon apparent stats and usage, and thus tiers are entirely arbitrary. Most people won't even bother thinking outside of the box when it comes to Pokemon. If others think it's good, it's good. If others thing its bad, it's bad. Thus, some Pokemon don't get to shine as often, beacuse people don't bother working with them. Look at how Sejun Park's Pachirisu stole Worlds, no one expected such a bad Pokemon (and a Pika clone to boot) to do so well. But beacuse Park is not an idiot, and people didn't have anything that could respond well to it, he won.
"Bad" Pokemon are only really bad beacuse people like to wear blinders and only focus on Pokemon that everyone else think are "good".