Quibbling over the origins of the Steele dossier misses the important point, which is the mindset of the previous administration and some of the people in its agencies. Regardless who kicked the ball, they ran with it. To understand the mindset, read about what happened to Flynn over his conversation with Kislyak. The previous administration tried very hard to find reasons to investigate people associated with the incoming administration. Investigations turned up nothing of consequence on the heart of the matter - "collusion". It has been a long and difficult pull (which reflects badly on the agencies) for the people who have been digging into the conduct of those investigations, and some of the information obtained so far reflects badly on the agencies (their motives and conduct). Once again, it isn't necessary to postulate a grand conspiracy where a bunch of little conspiracies by like-minded and -motivated people will do.
Trump, who ridiculously claims he won the 2016 election in a landslide, might merely be indulging his habit of self-promotion when he claims he won this one. But it should be tediously obvious that from his point-of-view one set of underhanded operations were undertaken against him. Why, then - in light of all the people who have openly objected to and vilified him for four years, including some Republicans - would it be unreasonable for him to believe that a second set of underhanded operations were taken against him during the 2020 election? To understand is not to excuse, but to pretend it doesn't matter is stupid to the bone.
To cite one infamous example, Comey repeatedly reassured Trump that the latter was not under investigation at a time when Trump was still very much a person of interest (or whatever term of art he fit at that time - subject, target, etc). If the FBI director lied to his president's face, why wouldn't the president reserve the right to suspect anyone?