It is a real reason, and treating it as ingratitude is what keeps people in a role for four more years than they meant to.
Boredom in the sense you describe is not a mood, it is information: your capability has outgrown the work. That has consequences beyond how the week feels.
Skills decay relative to the market. Eighteen months of variations on work you already knew is eighteen months where others moved. This compounds, and it is the part that is invisible until you are interviewing.
Your options narrow. The longer you do only what you can already do, the more your next move looks like the same job elsewhere.
It rarely stays neutral. People describing exactly your situation usually describe resentment a year later.
So the question is not whether it is a sufficient reason. It is what specifically is missing, because that determines whether leaving is the fix.
Be precise about it. Is it that the problems are too easy? That there is nobody to learn from? That there is no scope to own something bigger? That the technology is old? Those have different answers, and two of them are frequently solvable where you are.
Work that out before you start applying, or you will optimise for the wrong thing and be here again in two years.