If they didn't like her then they wouldn't have used her. It's possible they like the character but had priorities above making her & Steve's relationship one of the key elements to the plot. The fact 'Kate' became Sharon because of the Russos suggests they do like the character and would like to do something with her - eventually.
They used Stannis on Game of Thrones, but not many believe the creators of the show like him.
The thing about "Kate" becoming Sharon is that it calls to question why didn't M&M, who liked Sharon based on various comments, didn't include Sharon. She's a SHIELD agent. TWS is all about SHIELD. The idea that M&M didn't play on using her at all seems unlikely.
Maybe turning "Kate" into Sharon was a compromise.
At best they're willing to keep her around, with minimal development and use as they expend effort on Natasha and Steve's friendship, or Wanda being his protege. Perhaps leaving serious development to someone else when they leave the franchise.
They don't have forever. For all we know, there might not be a Cap at all in phase 4.
They seem to be focusing on the friendship between Steve & Bucky which, I'd argue, is the more immediately relevant issue given the way TWS & TFA played out.
And they seem to be allocating considerable time for Natasha again, and Wanda, and Vision.
I think, all things considered, Sharon has a pretty reasonable looking role in this, and its possible she'll be showing up for Infinity War too.
She's not showing up in Infinity War. Let's not kid ourself, here.
By all accounts, her role is just about bigger than her role in TWS. Talk about damning with faint praise.
The one thing the Russos are continually adamant about is that each character has an arc, regardless of whether they have 10 minutes or 60 minutes of screentime. Just because Sharon isn't in a ton of the movie doesn't mean what she actually does would be any less compelling or important.
Longer arcs tend to be more satisfying, though. I'd much rather see Sharon and Steve developing a rapport by working closely together than her just being super loyal for no reason and sending him emails.