Mike's aversion to Pryce's vehicle -- why was he so opposed?
I got initially the impression that Mike would simply be embarrassed to be seen in one although he does not seem to be the sort of person who worries about what others think.
But it turns out he was 100% right -- the hummer (or whatever) was directly responsible for the subsequent problems. Was this just instinct? Was Mike concerned that Pryce would look sillier than he already did to Nacho?
One objection I would have made in Mike's position was, it makes him easier to follow and if just by chance they drove past him on some other occasion, they would instantly notice him and then follow him home -- Pryce allowed them to skip that step by stupidly, oh, so stupidly, allowing Nacho to sit in his frigging vehicle while Pryce was not even watching. You can say Pryce is sort of an unsophisticated person but he actually seems just plain dumb.
The big, yellow Hummer with "PLAYA" vanity plates and spinning hub caps could cause a lot of things to happen, and none of them are good.
Mike was a cop and he knows perfectly well what any other cop would think seeing someone driving that car: drug dealer.
Mike knows that cops keep a lookout for such vehicles--in a later episode he calls the car a "blinking neon sign of a vehicle that says drug dealer". It is easy to spot and easy to follow. If the cops spot this huge yellow Hummer, they might follow it for a while and run the plates. If they see it is driving out to some isolated location (wherever he was meeting Nacho), they might decide to surveil the area and see if they can bust a drug deal. Simply finding out that the flashy drug dealer car belongs to a pharmaceutical company employee could be enough to spark an investigation.
Later, when Mike sees Pryce drive the hummer to the police station/courthouse, he indicates that the Hummer is what probably got the police interested in Pryce for the fishing trip they called him in for. It has put Pryce on their radar. Mike knew the minute he saw the car that this was a danger.
Pryce works for a pharmaceuticals company, but is just an IT guy who probably makes modest money. Those two facts put together with a ridiculously expensive and flashy car are what you call a major red flag. If Pryce is this careless with the car he drives, how careful could he have been covering his tracks where he works? Even the slightest police interest in Pryce could wind up with him in cuffs, and from there it is only a short hop to Pryce testifying against Nacho Varga and dragging the cartel (and Mike) into it.
The high profile vehicle might also catch the attention of other drug dealers.
Drug dealers can be extremely territorial and if they see this car riding around their neighborhood, they might decide to follow it and possibly confront Pryce (who is in no way capable of handling such a confrontation). They may also decide to follow the car to Nacho's exchange and try to rob Pryce after he has the money.
Mike also may have thought that Nacho would react badly to Pryce acting so recklessly and potentially drawing police attention.
Mike would rightly be concerned that the Cartel guy they're dealing with might see this gross disregard for keeping a low profile as dangerous and decide to just kill Pryce (and Mike!) and bail on the operation. Nacho does not react this way, but perhaps he should have. The car puts Nacho in danger and a lot of drug dealers would react badly to a business associate suddenly raising red flags that would get police attention. In Nacho's shoes, it might even have been what Mike would do.
A less immediate danger, but a danger nonetheless, is the possibility that the security people at the pharmaceuticals company will take notice of Pryce's unexplained affluence and investigate his computer logs.
We know from later episodes that Pryce acquired the pills by routing deliveries on the company computers. If the security people at his company are on the ball, they could take notice of the car and see that it does not add up, given his salary. This might cause them to look into his computer logs/history, which could lead to him getting caught. (It was unexplained affluence that caused real life spy Adritch Ames to get caught--he had co-workers over for a party and one guest reported to security all the extremely expensive things in his house).
And finally, yes, I suspect you're right that there's a pretty strong streak in Mike of being embarrassed about riding in such a vehicle.