So, like the title says, what are mountainous plains? Is this just non-forested mountains, or something else (like plateaus)?

My guess would be plains at a high elevation, as described at https://en.wikipedia.org/wiki/High_Plains_(United_States)#Geography_and_climate and https://en.wikipedia.org/wiki/High_Plains_(United_States)#/media/File:Level_III_ecoregions,_United_States.png or like the Tibetan Plateau, as described at https://en.wikipedia.org/wiki/Tibetan_Plateau.

Or, think the Chinese or Russian steppes. So, shrubs and weeds covering the ground with few, if any, trees present.

Yup, that would be my guess as well.

Plains that are above the treeline, so trees won't grow there.

Or another variation on the theme, a tepui.

https://en.wikipedia.org/wiki/Tepui

Which book did you find this in?

Honestly, it doesn't make any sense to me to describe something this way. Was it meant to be something like, "plains with a sprinkling of mountains", and the writer shortened it into "mountainous plains" accidentally changing the meaning?

If you wanted mountains without trees wouldn't you just say something like, "barren mountains", "scrub-covered mountains", or "grass-covered mountains" that would be far less confusing?

The description of Ord Mantell is in the Edge core.

The description of Ord Mantell is in the Edge core.

I read through it twice (page 361) and see nothing that says, "mountainous plains". Under the Terrain heading it simply lists, "mesas, plains, mountains, islands". Perhaps I need coffee and to read it a third time or it changed in a reprint?

Mountainous plains, nice contradiction heheh

I guess they mean uplands surrounded by mountains or something

The description of Ord Mantell is in the Edge core.

I read through it twice (page 361) and see nothing that says, "mountainous plains". Under the Terrain heading it simply lists, "mesas, plains, mountains, islands". Perhaps I need coffee and to read it a third time or it changed in a reprint?

You are correct. Mountainous plains appears to be from The Old Republic where I had gone for additional info on the world. I think I'll stick with the EotE terrain since it makes far more sense to me.

So, like the title says, what are mountainous plains? Is this just non-forested mountains, or something else (like plateaus)?

I'd assume that it's something like the plains around Mount Zion and Bryce National park. Desert, rocks and, plains - but at a high altitude.