Can you easily find friends' ghosts?

It IS possible to race against other "local ghosts" (ghosts from other player profiles on your Switch), but the procedure is a bit convoluted:

1. You need to add the other local player as a friend on the Switch's main menu.
   1. Both players need to have their Switch player profiles linked to My Nintendo accounts. (Mario Kart 8 Deluxe will prompt to you do this if you haven't done it already when you try to upload a ghost after finishing a time trial.)
   2. Then, from the Switch's main system menu, one player can add the other local player via their Friend Code. (As of the time of this writing, you actually do need to write down or photograph the other local player profile's friend code; the Switch won't "look it up" for you!)
   3. The friend then needs to accept that friend invite. (Since both player profiles are on your local Switch, you can just do this for them.)
2. Then, your local friend's ghosts will show up with an orange background (as in @Batophobia's answer) when looking at the lists of "Online ghosts" for a course, provided the ghost was uploaded (as MK8D prompts you to do after finishing a time trial).

Since my wife and I both have a switch, I decided to do some science. I linked my account as a secondary on her system, did some races, and uploaded the ghosts online. When looking to race and scrolling through online ghosts, friends seem to have orange backgrounds on their picture while your profile is yellow.

^{Note: The yellow bar is the selected option, it has no significance to your profile. It's how you choose which ghost you want to race against.}

Continuing the science, I got on my switch and checked the ghosts available.

^{Note: Again, highlight is the cursor for selecting a ghost.}

You'll note that in each, the friend user "Chrissy" is orange. However, only the same-system version of my account "Starkey" is visible. My system did not have my ghost data from her switch, even though my system had no other ghost data for me.

Just to test more options, I decided to do another track but not upload the ghosts. Unfortunately, I was unable to find a way to race against local ghosts. However, you can upload your ghost after the race or using the "Upload Ghost Data" option at the bottom.

Update

It seems like only your fastest ghost is kept. If you complete a faster time trial but want your friends to race your previous ghost, do not upload. You can still upload your faster ghost data later. Note that you can have 2 ghosts per track by switching between 150cc and 200cc during track selection.