I actually had this major insecurity myself once. Not just over plushies, but over collectibles and merchandise in general. It wasn't that I was afraid of not being manly, as I don't care about that stuff. It was more that I was afraid of people thinking I was some otaku or manchild, mainly my family.
Getting stuff like Rosalina merchandise (which I wanted to start collecting), especially the plushies, worried me most because of all of the creepy connotations that could come from it (I had a general insecurity over really liking the character for a long while). I've seen certain videos myself that were kinda sick, involving the amiibo and stuff. I knew I didn't want to do anything like that with anything I bought though. Rosalina just has a lot of high quality but rare merchandise, and I really wanted to collect it. My mind set is that I want to decorate my room, and later, my own living space, with gaming merchandise (and a bit of anime and other stuff too), based on characters I like and what not. I'm comparable to someone who likes collecting antiques, really. I just like having the stuff there to look at and show it to other people. I also like taking pictures of them, sometimes little dioramas and jokes. I enjoy photography, but I've kinda exhausted the things I want to take pictures of in my locality, so I take pictures of my merchandise now.
In the end though, how did I get over it? I just went against my "better judgement" and did it.
And now:
I don't think there is really any turning back now, not that I mind at all. And people actually don't bother me much about it. Heck, surprisingly, my dad supported it, he thought it was good for me as I was setting definite goals for what I wanted to collect and I planned my finances around it so that I could still afford to go to college. In general, I am probably the best with money in my family anyway.
And heck, even my personal wallet is merchandise now. But it's practical too!