Didn’t do any mountain biking but saw lots of trails with potential and I know there are companies that do mountain biking trips of different levels of difficulty.
It is a great country especially for wildlife. Tortuguero on the Caribbean coast is a fantastic place to visit, no mountain biking, and access only by boat or plane. The south east coastline on the Caribbean has lots of trails but unsure about use for mountain biking as some are in nationa parks.
The Arenal Volcano area is another great area with lots of unsurfaced roads and while I was there did see lots of mountain bikes on these roads.
The cloud forest in the interior is high up and the two roads up are long and rough and in the wet season can get very muddy requiring 4×4 to get there. There is a trail down from Monteverde, main town in the cloud forest, to the Arenal area that is only used by groups on horses. I’m sure this might have mountain bike potential.
The Pacific coastline is much drier than the interior and eastern side and probably has the better trails for mountain biking.