Dennis in Times up- "Well guess what pal, you are ugly and it doesn't matter how much you work out, its never going to change that fact."
Dennis in the its always sunny book published 3 years prior- "Mac for instance is also a 3 in ugly, so even if this exercise we're going to try fails, you'll have some company at your ugly parties. For the record, on the ugly scale charlie is a five, frank is a six, and Dee is a nine."
Dennis considers Mac to be the most attractive of the gang (not counting himself obviously). And I feel like this just further proves what an unreliable narrator he is. In Breaks up he tells mac he has nice biceps and beautiful lips (this is cannon in my heart even though the scene was deleted it still happened). He'd rather cut his limbs off than admit to something like that now.
Dennis has a boner for Macs ripped new body and it makes him so mad how hard he is for him and also so angry and jealous that he's not as ripped as Mac is.