is Cobalt mobile first responsive

Actually YES, but...

In general the output includes Twitter Bootstrap classes. So, a basic responsive behavior is already there. Did you checkout the DEMO page?

Of course, there are some areas where you don't get a 100% result out-of-the-box. i.e. Hence the two table-based list templates (default, AdvancedTable in extra template pack). So, if you create and display many fields in horizontal order ( >4 columns), you will get the usual crop on a small device. But that is a common limitation of tables. Maybe something you have encountered in Seblod as well.

You can overcome this issues by a responsive JS table script, and/or CSS classes to simply hide the less important data columns.

There is also the typical section menu bar. Which is based on Bootstrap Navbar and DropDowns as well... but it is not collapsing by default. It "floats". You can see the behavior on MintJoomla and Demo page.

In general, Cobalt is responsive... but you might need a specific appearance and style ... so, you may have to add custom classes on fields/labels (in config) and you may modify the default templates, or even create your own custom templates.

Overall, comparable to the Seblod stuff, IMHO. Although it has a different approach for the main template and where to use custom inline CSS etc.

PS:

1.) What was your complaint in regards of Seblod? (you mean the SEB1 template positions?)

2.) K2 needs template overrides. Joostrap templates incl. BS overrides for all K2 templates and module.