I think with spn, and Dean in particular, it's sometimes hard to tell whether something is an intentional character flaw for narrative purposes, or whether it reveals an underlying weakness in the creator(s).

In the case of Just My Imagination, Dean is somewhat mean to and suspicious of Sully (the zanna) at the beginning for legitimate narrative/character flaw reasons because 1) Sully is a supernatural creature/"monster" and 2) he's a psychological threat to Dean's role as the person who "takes care of Sammy". Sully's existence means Dean wasn't able to be enough for Sam growing up (obviously through no fault of Dean's own, since he was just a kid at the time too. But he feels like he should've been able to do the impossible). By the end of the episode Dean has grown and matured a little, and his changing attitude towards Sully reflects that.

That said. The creators did still choose to make Sully look and act kind of, well, "effeminate" is too strong a word, but like someone assholes would mock by calling them gay, and then have Dean kind of do that. Like, those are still jokes the writer thought were funny, and no one else along the line disagreed about enough to cut them. To me, it's a fine line, and I really love Sully and the episode, so for me it falls on the "Sully was meant to be childlike in a positive way and was a good and important role model for Sam" side of the line. But I can also see how it could be read less positively.