Why did the Uchiha weaken so much after coming from Indra?

[Zee U]

The fact that Hashirama mentioned them and not the other clans, and the fact that Madara was surprised to find they joined them suggests they weren't a nobody weakling clan either.

Or maybe they were the only that joined them at time, no? Why do you assume other clans joined them? Also them not being a nobody doesn't put them on the heavily hyped Senjus/Uchihas.

Not disputing the last part. You are doing what you said everyone else is doing, you are basing the entire clans strength based on Ashura/Indra transmigrants. The only reason we are to believe these 2 clans were the strongest was because Kishi only mainly revolved the story around these two clans.

I'm not basing it on the transmigrants. Uchihas were strong when Sasuke was still a kid (heck, they were confident they could take on Konoha. If they had no chance from the beginning why would they even bother?) Uchihas were strong when Madara was still a kid. Same can be said for the Senjus.
Kishimoto made it painfully obvious that those two clans had something the others lacked, even when the transmigrants weren't at their peak. Heck, teen/adult Madara and Hashi lead the two main forces. What did they have at time? MS, maybe even 3T? Piss poor Mokuton?

Turns out they are the only clans with Otsusuki Blood. apparently so are the Hyuugas, then there are the Uzumaki who the entire Shinobi world turned against out of fear of their sealing jutsu alone.

Uchiha/Senju being the strongest clans are debatable

Uzumakis are a question mark in my opinion, sealing jutsus are highly situational and Kishimoto hasn't expanded much on them. Who knows, maybe they had OP ones like being able to prevent chakra kneading. I wouldn't make assumptions.

Uchiha and the Senjus are heavily portrayed to be the strongest, the main, the best clans out there.