There is no precedent for someone stronger than Luffy joining the crew so how is it the entire manga?
Zoro was much more famous than Luffy when he joined, and when Robin joined her bounty was nearly triple Luffy's.
On the opposite end, Usopp is weak as all get out and that wasn't a factor at all in his joining. Brook was asked to join on the spot, long before he had any actual showing of fighting capabilities, backstory or bounty.
Zoro and Luffy have never had a decisive fight beyond "they're basically equal", same with Zoro and Sanji. The crew pretty much concedes that post gears and Haki Luffy is the toughest, and its in Luffy's personality and desire to go against the top bad, but other than that? On Fishman Island Luffy and Jinbe had a qick exchange to show they were about equal, with Sanji in the middle. They don't care. (And subtlety that was lost int he Viz translation then is Jinbe switched from "Luffy-kun" to "Luffy-san" after that.)
Luffy was fine with Sabo taking his place at the tournament. And he was fine with tag teaming wit Law against Dofla. And he was cool with letting Crocodile and jinbe out of prison to help him out.
The Supernovas are generally treated equally. Luff, Kidd and Law at the top maybe, but they're in general treated as a whole group of equals, regardless of feats or bounties.
Toss in time and time again how we've seen a weaker character overcome a stronger one through wits or trickery. Like the time Perona instantly destroyed the entire crew except for Usopp.
Strength doesn't matter to Luffy. And it doesn't matter to Oda. He toyed with power levels ever so briefly among the CP9, and then immediately dropped it and never brought it up again… and in fact made it immediately irrelevant by giving two members new devil fruits right after that.
If Oda cares at all, it's to the degree that he held off Jinbe's joining until he had a chance to inflate everyone else's bounties a bit, and to not have three warlords in the same fight on Dresserossa.