Surprisingly, no. Before seeing the film I thought that was going to be case, i.e., I was going to have difficulty differentiating Mike from Richie because of the actor, the freshness of Stranger Things, and how seemingly similar the two roles appeared to be. But no, to be perfectly frank it became easy to forget that Finn (Richie) was in Stranger Things.
Sure, the connection between Stranger Things and It, along with Finn's casting, is hard to ignore (unlikely his part in the former got him a role in the latter). But the thing is, he was so different as Richie than he is as Mike, all in all I wasn't reminded of his involvement in ST. However, having seen some recent interviews with him, what I did have in mind while watching IT (seen it more than once already) was how in reality he no longer looks and sounds as young as he did in both the show and film. 🙂