I am not very sure about that. Starmie has a very high base speeed of 110, outspeeding almost every stealth rock setter and spike setter in the tier. Starmie isn't also harmless, and can 1Hit KO common stealth rock users.
0 SpA Starmie Hydro Pump vs. 4 HP / 0 SpD Excadrill: 338-398 (93.3 - 109.9%) -- 62.5% chance to OHKO(without special attack investment with Analytic)
0 SpA Starmie Hydro Pump vs. 0 HP / 0 SpD Landorus-Therian: 288-338 (90.2 - 105.9%) -- 37.5% chance to OHKO(with Analytic)
Defensively,
252+ Atk Landorus-Therian Earthquake vs. 252 HP / 4 Def Starmie: 222-262 (68.5 - 80.8%) -- guaranteed 2HKO after Leftovers recovery
(222, 225, 228, 231, 232, 235, 238, 241, 243, 246, 249, 252, 253, 256, 259, 262) (Starmie will still win because of its faster speed)
252 Atk Mold Breaker Excadrill Earthquake vs. 252 HP / 4 Def Starmie: 192-226 (59.2 - 69.7%) -- guaranteed 2HKO after Leftovers recovery
(192, 193, 196, 198, 201, 202, 205, 207, 210, 211, 214, 216, 219, 220, 223, 226) (Once again, starmie will win because it is faster)
Because of this, starmie is a very reliable rapid spinner and isn't bad defensively either. Starmie is very fast, and can tank at least 1 hit before dying. It is much better that all the other spinners, who are slower than most offensive pokemon, little offensive presence, etc. Starmie is BEST spinner in the tier, we can't deny that.(ALthough it is true that defog is the vastly superior option)
My only issue about defog is that it also removes hazards from the ENEMY'S side, which is not desired. There are only few good ghosts types(that are commonly used) in the OU(Alolan Marowak, Mega Sablelye, etc), so blocking rapid spin isn't that big of an issue(in the OU anyways. Ubers has Mega Gengar, making spinners unreliable and defog much better)
In conclusion, Starmie IS severely underated and IS the best spinner in the tier(far ahead from the rest). "Starmie isn't much better" is a bit inacurrate. "Starmie is better" sounds more accurate to me.