You can't look at it like that. The Ten-Tails is weaker than Kaguya, even if it's a mutation.
Using that logic, Jinchuuriki Obito and Jinchuuriki Madara would be stronger than Kaguya as they sealed the almost-complete Ten-Tails and gained its power.
You may argue we're talking about the complete Ten-Tails here, but the difference isn't huge compared to the one Madara summoned.
Hagoromo and Hamura should have been quite superior compared to the Ten-Tails.
In my opinion, Hagoromo only got a boost of around 20% by becoming a Jinchuuriki.
Well good point, but let me show you another view at it.
Let's start from the begining: Kaguya consumed Holy fruit and gained all the holy power for herself, let's call her Prime Kaguya
Later on Kaguya gave birth to two boys Hagoromo and Hamura, who inherited some power of Holy fruit.
I would assume Hagoromo and Hamura took TOGETHER around 60 % of Kaguya's original power.
I assume that because Hagoromo and Hamura spread some chakra to mankind then and still they had to be enough powerfull to beat weakened Kaguya who merged with Shinjuu to gain Ten tails form.
After beating Ten tails they sealed it into Hagoromo, so he would have 40% of Kaguya's Prime power + 30% of Kaguya's prime power in shape of Ten tails.
In other words Juubi consist of only 40% of Kaguya's prime power just little above each of her sons.
Madara as Juubi Jinchurki had 25% of Kaguya's Prime power(Almost Complete Juubi) + himself(25% of Kaguya's prime power) so as result 50%
Obito as Juuubi Jinchurki had 15 % of Kaguya's Prime power(Incomplete Juubi)+ himself(15% of Kaguya's prime power) so as result 30%.
Kaguya who appeared after taking over Madara has same amount of power as Juubi so 25% of her original power, then absorbed chakra from living being caught in IT so around 50% of her original power.
Later on she powered up, once again most likely being little above 70% of her original power.
Summary:
Prime Kaguya: 100% of holy fruit power
Kaguya after birth of Hagoromo and Hamura: 40% of holy fruit power
Yang Hagoromo: 30% of holy fruit power
Yin Hamura: 30% of holy fruit power
Jin Hagoromo: 70% of of holy fruit power
Post Jin Hagoromo: 35%-40% of holy fruit power
Kaguya after revival: 25-30% of holy fruit power
Kaguya after absorbing chakra from IT first time: around 50% of holy fruit power
Kaguya after absorbing chakra from IT second time(last power up): slightly above 70% of holy fruit power