Overall, I like the streamlined nature of the rules of Mutant Crawl Classics RPG, but sometimes the streamlining erases important details. One such detail is the nature of certain passive mutations. Mutations by their nature are things that stray from the norm. They are abnormal. So, why do some passive mutations have the possibility of yielding results that are indistinguable from the normal non-mutated range? Here, I am referring especially to mutations that have a chance of altering a character's ability scores, such as Heightened Intelligence. Unless the player rolls 20 or higher, the ability score improves by merely +1, +2, or +3. In many cases, the character might not even gain any tangible benefit. If the ability score were 9, for example, a +3 improvement wouldn't even boost it beyond the average range.
I have an alternative.
When rolling passive mutations such as Heightened Agility, Heightened Stamina, Heightened Strength, Heightened Intelligence, Dual Brain, etc., ability increases refer to the ability modifier rather than the ability score. The ability score is then increased to the threshold of that ability modifier.
For example, a mutant, manimal, or plantient with Intelligence 6 (-1) who acquires the Heightened Intelligence result, "The mutant's Intelligence score is increased by +1," now has Intelligence 9 (+0) because 9 is the minimum ability score at which the modifier becomes +0.
Granted, having one's low Intelligence raised to the dizzying heights of average seems at odds with the term Heightened Intelligence, but at least this method offers a guaranteed tangible benefit with each increase.