The grammar pattern のももっともだ is used to express that something is perfectly natural, understandable, or no wonder given the circumstances. It conveys the speaker's acknowledgment that a feeling, reaction, or situation is entirely reasonable and justified.
The structure breaks down as: Verb/Adjective (plain form) + のももっともだ. The の nominalizes the preceding clause, も adds the nuance of "even/also," and もっとも (最も/尤も) means "reasonable" or "natural." Together, the pattern means "it is only natural that ~" or "one has good reason to ~."
This pattern is used when the speaker empathizes with or validates someone's feelings or actions. It often appears in contexts where you are explaining or justifying why someone reacted a certain way.
It is commonly preceded by a reason or explanation, often with expressions like あれだけ, あんなに, or causal connectors like のだから.
In more formal or polite contexts, もっともです is used instead of もっともだ.
This pattern is similar to のも当然だ and のも無理はない, all of which express that something is understandable or justified. However, のももっともだ carries a somewhat more formal or literary tone and often implies the speaker's reasoned agreement with the situation.