Seriously, yes it is the one and only con with an otherwise excellent RTA. The tiny spots of condensate are nothing really, it doesn't leak and the size is just brilliant.
The top cap has a step the function of which is to accept the 510 drip tip adapter ... if they remove the step to flatten it, no change to the overall height, there is room for a top fill port. One fill hole, a second hole to allow air out. Instead of an adapter they could supply two top caps, one for the Goblin's glass DT and another for a standard 510 DT.
If they change nothing else they would make the Goblin Mini a stellar product ... selling the new caps on their own would make the current version upgradable. Steam Crave have done that with the Aromamizer RDA-H.