As a hiring manager who looks at a lot of resumes, I think your hunch is correct. I see a lot of junior to mid-level developers that primarily work in JS/TS/React. That market is heavily saturated and I regularly read posts on LinkedIn from these people about how difficult it is to find a job.

The candidates that stand out from the pack are those with some tangible production backend experience in languages like C#, Java, etc., combined with SQL experience. Strictly in terms of marketability, I do believe getting real-world backend experience greatly increases your value.