1A conjunction that qualifies or slightly contrasts what was just said—'but then', 'although', close to a soft 'however'.
彼は来るだろう。もっとも、遅れるかもしれないが。
He'll probably come. That said, he might be late.
その意見には一理ある。もっとも、全てを鵜呑みにはできない。
That opinion has some merit. That said, we can't take everything at face value.
彼女は仕事ができる。もっとも、経験はまだ浅いが。
She's capable at her job. Although, she still doesn't have much experience.
2Reasonable, natural, justified—used to say something makes sense or is fair, as in 'もっともな理由' (a valid reason).
それはもっともな意見だ。
That's a reasonable opinion.
彼が怒るのももっともだ。あんな失礼な態度をとられたら誰でも怒る。
It's only natural that he's angry. Anyone would be, treated so rudely.
もっともな理由があれば、上司も納得してくれるだろう。
If there's a good reason, your boss will probably understand.