js hype isn't temporary, I think too many years have gone by since "js everything" movement started to consider it a hype, even though I very much disagree using js for large server side apps.
It's ubiquitous because it's the only real client side language. So, it doesn't necessarily mean that you need to bound yourself to MEAN stacks. That's the idea of stacks, you should be able to replace any letter of the acronym with a technology of your choice. That you don't have to buy an engagement ring for any of them.
So, invest on the js front end side, that's a no brainer. On the other hand, if you want to do a MEAN stack, go for it, it's very popular. You can always learn something like postgresql instead of mongo. And you can learn Java or ASP.NET for the server side language.
Any of these choices will be more than ok.