Unfortunate that OP's criticism just boils down to 'rap is crap'. I have plenty of good reasons to hate basically every music change the show has made lately. And I'll just get it out of the way, that, yes, this is an extremely pet-peevish list of grievances and I am a loser with too much time on his hands.
The theme song makes me literally cringe. First of all, 'REEEEMIIIIIIX' is just stale and cliche as fuck. It's what unfunny people do when they're doing their 'I'm being gangsta but I know I'm hopelessly white!' schtick (which, I repeat, is stale and cliche).
Second, it's so circle-jerky. The theme we all know and love perfectly outlines what the show is, very simple. This is a masturbatory 'hey remember when we did that movie? Remember when June said that? Remember the Odd Life of Timothy Green episode? Eh? Eh?'
Third, and worst of all, it's so. fucking. long.
This basically applies to the Second Opinions song, as well. A nice little interstitial horn will do just fine, absolutely do not need 'and now here's Paul, June & Jason doing a funny song!'
I'm operating under the assumption that the show is Paul's baby and as much as I love him, he has THE WORST instincts when it comes to music. Go back and listen to the really early episodes, where the music was just the worst kind of generic shit rock.
I'm done (being a complaining asshole, not with the show).