Could it have also been a bit of jealousy on Tony's part? Junior was more of a father to Tony than his own father but he never really got the love he wanted from Junior. "He never had the makings of a varsity athlete." Maybe seeing this asshole, who was about his age, being his Uncle's main guy, someone Junior clearly respected more than Today, hurt his feelings in some way, or being a dick to Mikey was a (subconscious) way of getting at Uncle Junior.
I see what you're saying, Tony was that vain where he would project that kind of subconscious aggression. I still think it really had nothing to do with Junior. Tony held no ill-will towards anyone else in Junior's crew, he broke Bobby's balls for being the only guy fatter than Tony same as he did Vito. He had Philly Spoons killed for spreading rumors. That being said, the hate for Mikey seemed like it was shared by all. There certainly was nobody who ever praised him while he was still alive.
Also, I can't imagine there was any jealous in regards to Mikey being Junior's #2, Tony got Johnny Boy's entire crew, realistically bigger and more profitable than Junior's. Tony had New York's ear, as well as every other captain. In honesty I really don't think any of it was jealous towards Junior, however I like your thoughts that the general disrespect could easily be attributed to that, it may not be jealousy but I'm sure Mikey being affiliated with Junior makes him that much more unlikeable to Tony versus if he were Ray Curto, Larry Barese, or Jimmy Altieri's #2. In that scenario Tony would likely smile through his teeth at Mikey but definitely not disrespect him in the same fashion as he does as Junior's right hand.
