"Flash" on a styrene kit is the result of at least one of two causes: Most think of flash as the result of poorly mated tooling halves, which of course can be true. However, the biggest cause of flash in a plastic model kit, as told to me by a production engineer at AMT Corporation years ago, is improper temperature control. Styrene is very much like pancake syrup in this respect: The hotter you heat it, the thinner the consistency, and not hot enough makes for a much thicker "syrup" of styrene. Too much heat, the styrene gets very thin, and can flow out in the minute clearances between the parts of a mold, resulting in flash. Too little heat, and the plastic may well not flow as it should through all the sprues and injection passages, which results in "short-shot" parts. (Many of use have seen both!).
It is a measure of quality, but more in terms of production quality, rather than a severe defect in the tooling,
I worked in Revell-Monogram Tool Engineering from 1992-2005. I'm not a molder, but have spent plenty of time around molding machines and kit tooling. There are so many possible causes for flash that it's not just a simple answer. Temperature control is one cause, another big one is because of mold filling problems. Getting any mold to run well is an art, and it can depend on how much time/effort the person dialing in the machine is willing/able to put into it. Sometimes filling problems can be worked out in the machine, but lots of times old molds probably didn't run well when they were new. We'd spend a lot of tool room changing stuff like gates and runners to try to get stuff to fill without having a ton of flash. Flash can cause problems with the mold too because chunks of it can get stuck between the mold halves and get "smashed" causing further mold damage/problems.