Yes - UV protection (not that a cockpit will get much UV exposure in use) and what you refer to as 'beauty-reasons" (which may be offset by what becomes a Very Slippery Surface when wet once varnished, to say little of ease of repair should that become necessary; varnish will have to be removed to add epoxy again) are the two reasons to proceed with varnishing.

Were I you I'd leave it as-is for the time being. I'd concentrate on doing a bang-up varnish job on the outside where it's necessary if your epoxy's otherwise unprotected from UV exposure, then get out and enjoy using your new watercraft!

Sanding the inside of a cockpit before varnishing may not be entirely necessary, in fact it may not be a good idea. Access to the bits out of reach, using a powered sander, will be tricky at best. If the object is simply to 'de-glaze' the epoxy surface – the better to create a surface that will readily bond with the epoxy – simply roughening it with ScotchBrite-style non-woven abrasive will be sufficient, easily done by hand. And this stuff more readily conforms to corners than sandpaper ever could so you can do a more thorough job with less effort.

I doubt a varnished interior will be any easier to clean than bare epoxy, and varnish won't offer much protection from abrasions from use, so there's that too. But I'm no kayaker so my opinions are conjecture at best; I'll leave it to the dedicated paddlers out there to give you their thoughts and suggestions as to what you might do next.