Trigger Warning
This answer contains quotes from the Bible which depict violence.
Why is an evil god seen as less likely than a good one?
Your assertion might be difficult to prove.
Here is a list of 32 Evil deities (there may well be more than appear on this list).
However, if we for a moment take it as true that an evil god is 'seen' as less likely than a good one (perhaps this is more likely to be true in those cultures in which Christianity dominates), then at least two reasons seem apparent:
Most people who ascribe to Christianity are brought up to believe that the Christian God is perfect, infallible. Despite the fact that many of God's biblical acts and attitudes are appalling by most secular human standards (see 'The 10 Worst Old Testament Verses by Dan Barker](https://ffrf.org/publications/freethought-today/item/26141-the-10-worst-old-testament-verses), Christians tend to ascribe goodness to them because it would be 'irrational' to ascribe unjustified malice to a 'perfectly good' being.
We have a very strong interest in believing that such a powerful entity - an entity with the capacity to cast us into infinite hell for finite crimes or 'sin' - is a good and just god that would not hurt us on a whim.
Why is an irrational god seen as less likely than a rational one?
If this is true, it is difficult to provide insight without having knowledge of the roughly 18,000 gods that humans have worshipped at one time or another, but by again looking to Christianity, the world's most dominant religion, we see that its 'perfect' god can reasonably be viewed as irrational upon inspection of the Old Testament, despite the fact that many if not most believers would probably claim that it is impossible for God to be irrational.
As an example:
"The Lord said to Satan, 'Have you considered my servant Job? There is no one like him on the Earth, a blameless and upright man who fears God and turns away from evil. He still persists in his integrity, although you incited me against him, to destroy him for no reason.' " (Job 2:3 New Revised Standard Bible)
and also...
"The people of Samaria must bear their guilt, because they have rebelled against their God. They will fall by the sword; their little ones will be dashed to the ground, their pregnant women ripped open." (Hosea 13:4, 9, 16 New International Version)
To many if not most people, punishing innocent children for the 'crimes' of parents would typically be deemed irrational.
In short, a person's tendency to view a deity as evil or irrational is likely determined to a large extent by how invested they are in a text which describes such a god as good and rational.
Is a good god more likely than a bad god?
There is absolutely no way to tell, for we have no reliable evidence that a god or gods exist.
