のももっともだ is a combination of の, も, the な-Adjective meaning ‘reasonable’, or ‘plausible’, and the auxiliary verb だ. In many cases, the interpretation will be ‘it may well be (A)’, ‘it’s only natural that (A)’, or ‘there’s no wonder that (A)’. However, 尤（もっと）も itself is simply an adjective that affirms that the speaker understands the thought process behind (A), and that it is logical to them, regardless of whether or not they actually agree with it themselves. Due to this, ‘it may well be (A)’ in English is actually a very close translation of the feeling behind the expression.
のももっともだ will appear with verbs, い-Adjectives, な-Adjectives, and nouns. In the case of nouns, the の will be dropped, as the noun itself will become the expression being marked as the focus.

• そんなことを彼女（かのじょ）に言（い）ったのか？彼女（かのじょ）が怒（おこ）るのももっともだ。
  You said something like that to your girlfriend? She has every right to be angry.
• 彼（かれ）はいつも夜（よる）遅（おそ）くまで残業（ざんぎょう）をしているから、いつも眠（ねむ）いのももっともだ。
  He always works late at night, so it's no wonder he's always sleepy.
• 彼（かれ）は高校生（こうこうせい）の頃（ころ）から毎日（まいにち）日本語（にほんご）を勉強（べんきょう）をしてきた。上手（じょうず）なのももっともだ。
  He has studied Japanese every day since high school. It is no wonder he is so good at it.
• 親（おや）の反対（はんたい）ももっともだが、俺（おれ）はもう俺（おれ）のやり方（かた）でやると決（き）めたから親（おや）の意見（いけん）に合（あ）わすつもりはない。
  My parents have a good reason to object, but I've already decided to do things my way, so I'm not going to conform to my parents' views.

Due to のももっともだ being used when the speaker is expressing that they understand the underlying reasons behind something, phrases used for explanation such as から or ので will often be seen in the same sentence.