I assume you speak of Diablo. And yes it absolutely and only depends on how fast it is growing (the XP in relation to Monster Lvl). It grows not fast enough. So they had to force it to give nothing, so old levels are obsolete. If instead the XP would be multiplied each level with 2, the numbers would a) grow so fast that they could not hold it in a standard int variable and b) the older levels would become obsolete without force
My point was, that it does not matter, once you reach your own threshold of the diminishing return vs. time spent. And this game combines the reset-increment seen in other idle games with leveling your girls outside the time bonus and not increasing your reset gain substantially the higher the girls get. From 0,5 to 9 over 10 girls? This is laughable for a idle reset game. Plus the time needed for resets is not decreasing because of fake timers and filming scene timers.
And because of pvp you do not even need to compare your time bonus with other player's. Instead your best leveled girl. And leveling a girl gets its own complicated diminishing returns, because oh no no no, you do not compare the level of the girl, but the practiced stats of the girl. The practiced stats are dependent of the lvl of the girl.
So eventually you need a higher tier girl, because you can get her to a higher lvl, wich allows her to get higher stats in a reasonable time.
Maybe it is because English is not my native language. But I think it is rather because I played other idle games like clicker heroes and Zombidle. I know diminishing returns only as a mathematical concept, resulting in certain bonuses becoming obsolete to spend resources on.