のももっともだ

のももっともだ says that someone's reaction is reasonable: "it's only natural that", "no wonder". The clause in front ends in the plain form and の turns it into a noun; a な-adjective or a noun takes な before の.

• × 不安のももっともだ → 不安なのももっともだ

もっとも here is the adjective "reasonable, justified". Its kanji is 尤も, but it is almost always written in kana; it is a different word from 最も, "most".

The reason usually comes first, ending in のだから, から or a conditional such as たら. The polite form is のももっともです.

のももっともだ, のも無理はない or のも当然だ

All three mean "no wonder". のももっともだ judges someone's feeling, opinion or reaction as justified: the speaker agrees with it. のもむりはない無理はない sympathises – the reaction is understandable. のもとうぜん当然だ says a result follows from its cause, and it is the one to use for an outcome rather than a reaction.

のももっともだ is the stiffest of the three; in conversation のも無理はない and のも当然だ are more usual.

On its own, もっともだ judges what someone says: きみ君のいう言うことはもっともだ, "what you say is fair". At the start of a sentence, もっとも is a different use: "mind you", adding a reservation to what came before.