Not really... but I suppose it all depends... mostly on age? but then again old people are allowed to be in love also... just seems someone younger would post little hearts and such... but who cares? Post what you feel and post what you want. People will criticize you anyway! but remember not to care when and IF they do it out loud. :p! That's how you do it! :D!
Immature? No, it's not. Rather, it woulld be immature to do all of that then proceed to brag about your relationship like you've found the meaning of life. It's okay to post photos, but be humble and modest. No sense making enemies lol. Of course, there will be jealous and hateful people that will complain that you are an attention bragging whore, but don't let that get to you as it's the way of the internet :).