Being popular has nothing to do with it, being a politician has nothing to do with it, what others may or may not have done has nothing to do with it.
There were a whole range of specific actions that occurred that resulted in an investigation as it should.
One of the FBI lawyers involved plead guilty to a felony false statement charge. They were literally making up pretexts to spy on the Trump campaign. I think the issue here is when you talk about "legitimate" you mean that you are happy enough to see the FISA system used as an internal political club. There is no way that it is being used legitimately, we're in the opening stages of a process where every single incoming US president is going to have their communications examined by the incumbent's administration. Or at least when the party in power changes.
There isn't anything special about Trump here. He turned out not to be doing anything suspicious, or they'd be going after him for it (or have gone after people in his campaign). Ergo, any clean candidate is probably suspicious enough to warrant surveillance.
They bought this system in notionally to fight terrorists. It has in short order turned out that "terrorists" include someone who can be described as a relatively popular (!) Republican candidate. Are you contending that this is legitimate use of the foreign intelligence apparatus?
Most US presidents have dealings with foreign countries. The Biden family are famously involved in Ukraine (& others), Trump was all over the place including Russia. Obama was probably just domestic but the Bush family have a lot of interest in oil which is going to involve foreign contact and the Clintons are closely involved in US foreign policy. There'd be a case for spying on almost every president in the last 30 years!