Credits:This visualization was made using this kaggle dataset. The Kaggle dataset is based on data scraped off of MyAnimeList and the ratings/score is a 1-10 scale by it's users. The tools I used were plotly and pandas in python to generate my visualization.Disclaimers:The portion of Mecha Anime (blue line) is based on the anime that begin airing in that year, anime that span multiple years may not be fully represented in my visualization. Additionally, as u/amatulic pointed out, the total number of anime produced each year is increasing so the number of Mecha anime is growing but the share of total anime that Mecha makes up is shrinking. The general trend of scores rising is not specific to Mecha anime although I was unable the show that in this visualization. Also, sorry about how plain the graph looks, I am still just beginning in data science.

Explanations/Insights:Especially up to the mid-1980s, the popularity/quantity of Mecha anime released seemed to lag behind the general ratings that it received where the red line can predict each peak and dip in the blue line. After a steep drop in ratings in 1987, Mecha anime only made up 15% of all Anime released each year and continued to decline despite scores generally increasing. Potential reasons for the loss in popularity are likely from the genre being exhausted of its good ideas continuously raising the bar and removing potentially great Mecha anime as cliche and repetitive.

Now its more of my opinion but I think Isekai and fantasy will see somewhat similar things in the future although the decline may be slower due to it being a much broader genre with more ideas to exhaust.