Hi guys,
Mostly people who run for instant gratification turn out be losers in the end.
I find this sentence OK, although I'd put a comma after 'mostly'.
With adverbs, there is often a lot of flexibility about where to positon them in the sentence. eg
She walked into the room slowly.
She walked slowly into the room.
She slowly walked into the room.
Slowly, she walked into the room.
All of the above are OK, although you'd probably hear #1 most commonly.
Very generally speaking, moving the adverb towards the front gives it more emphasis, although this can also be done in other ways, such as by tone of voice.
Best wishes, Clive