More generically, the whole layouting of the mention is hardcoded:

• as far as I can see paddings sizes are also hardcoded, I am not sure it should be the case (since it's for an inline box, I think I would expect the padding to be dependant on text size, but size of whitespaces is a complex topic, some should be text-dependant, some should not)
• more generically, the style of the mentions is probably better off as a copy of the style of bootstrap badges or labels, at least in terms of border radius & probably, padding.

See also https://getbootstrap.com/docs/3.3/components/#labels and https://getbootstrap.com/docs/3.3/components/#badges .