Translation:She is less beautiful than her mother is.
Lei è meno bella di quanto lo è sua madre = Lei è meno bella di sua madre
"quanto lo è" = in this case "quanto è bella" (But saying "Lei è meno bella di quanto è bella sua madre" would have been a bit too repetitive.
"Lo" can substitute something that was already mentioned.
"Te lo ho già detto" I already said that to you
"Quanto lo hai pagato? How much did you pay that?
"Tu sei più brava di chi lo ha già fatto." You are better than who already did it.
"Lo" replaces something already said, or implied, it's often replaceable with "it" and "that". OFTEN... I looked for some good source on the topic, but didn't come up with a lot of stuff, if somebody does it would be great.
Lo voglio quanto lo vuoi tu - I want it as much as you want it
"ne" is for things, "lo" for actions, probably.
But don't focus on the "tanto... quanto" for this topic, or you won't find a lot of stuff. If you find some useful link, please tell me, because thinking from an Italian point of view doesn't help me to find anything good now.
Help, marziotta, I'm utterly confused now. I didn't understand why you wrote "TE lo ho già detto" as one of your examples (rather than TI, as I thought I'd learnt), and I looked it up and found that indeed it's "te l'ho detto" but "ti ho detto" (minus the 'lo')... can you explain, please? Thank you very much.
"In many cases both a direct and indirect object pronoun will appear in the same sentence. When such occurs, the indirect object pronoun almost always precedes the direct object pronoun. Note that mi, ti, ci, and vi change to me, te, ce, and ve when followed by a direct object pronoun."
Germano, Joseph (2009-08-25). Schaum's Outline of Italian Grammar, Third Edition (Schaum's Outline Series) (Kindle Locations 6065-6067). McGraw-Hill. Kindle Edition.