In some ways this would be a brilliant choice.
Romney once famously said that Russia was the real enemy. That makes him an instant band-aid for any further early term speculation about the administration becoming too close to Putin. It might also be a calculated message to Moscow — we want better relations, but not the chance to kiss your butts.
Also, Romney is quite inoffensive to most foreign leaders compared to almost anyone else Trump could name to this post, and that might allow things to calm down long enough to get difficult new relationships accepted and moving forward. Any idea that the U.S.A. is in a position to dictate bilateral terms with various countries is probably very popular in the Trump base but demonstrably inaccurate and doomed to failure (all that would happen if somebody tried that would be a freeze in the status quo followed by a four year wait for change, which also might please some but I don’t see the point of even having a Secretary of State if that’s the chosen path).
Another good thing about hiring Romney is that you know he’s going to feel bummed about Trump winning where he lost, so you can rub it in about once a month.
Also, being new to politics, maybe Trump thinks you have to be a failed presidential candidate to become Secretary of State. /s