No.
+6 252+ Atk Smeargle Extreme Speed vs. 252 HP / 252+ Def Suicune: 145-172 (35.8 - 42.5%) -- 94.5% chance to 3HKO after Leftovers recovery
4 SpA Suicune Scald vs. 248 HP / 0 SpD Smeargle: 148-175 (47.2 - 55.9%) -- 79.3% chance to 2HKO
+2 252+ SpA Smeargle Boomburst vs. 0 HP / 0 SpD Mega Aerodactyl: 102-120 (33.8 - 39.8%) -- guaranteed 3HKO
244+ Atk Tough Claws Mega Aerodactyl Aerial Ace vs. 248 HP / 8 Def Smeargle: 321-378 (102.5 - 120.7%) -- guaranteed OHKO
Of course you could run specific coverage to beat those mons (this is Smeargle, after all) but then I'll just grab something else out of A-rank to beat you. When I can just randomly grab pokemon out of the top of the viability rankings and they effortlessly counter your set, your set probably isn't any good. Getting a sleep-inducing move doesn't make up for these terrible stats.
You forgot to take life orb into account.
Anyway, I actually think that grass-type coverage could be the best for bellydrum smeargle, as it easily beats rock types and bulky waters. As well as doing this:
+6 252+ Atk Life Orb Smeargle Power Whip vs. 252 HP / 252+ Def Suicune: 377-445 (93.3 - 110.1%) -- guaranteed OHKO after Stealth Rock
Not everyone runs defensive pokemon anyway, and Smeargle easily takes care of a lot of semi-bulky offensive threats.
+6 252+ Atk Life Orb Smeargle Extreme Speed vs. 0 HP / 0 Def Hydreigon: 316-372 (97.2 - 114.4%) -- 81.3% chance to OHKO
+6 252+ Atk Life Orb Smeargle Extreme Speed vs. 0 HP / 0 Def Salamence: 347-409 (104.8 - 123.5%) -- guaranteed OHKO
+6 252+ Atk Life Orb Smeargle Extreme Speed vs. 0 HP / 0 Def Shaymin: 289-341 (84.7 - 100%) -- 81.3% chance to OHKO after Stealth Rock
Pair it with Magneton, who happens to be very underrated, and you have an impressive duo.
Now about to Boomburst Smeargle, I probably overrated it, but if you remove priority users it could still be very dangerous:
+2 252+ SpA Life Orb Smeargle Boomburst vs. 0 HP / 0- SpD Hydreigon: 308-363 (94.7 - 111.6%) -- guaranteed OHKO after Stealth Rock
No coverage move is really notably better than another. Blue Flare takes care of most steel types, thunder gets mega aerodactyl, seed flare destroys rocks, etc. It’s not different from most sweepers, really- there will always be hard counters. That’s why you have teammates.
And you have to remember, Smeargle may not always sweep, but it is still incapacitating one threat with Spore (or Dark Void).