Jesse Pinkman has yet to appear in the flesh during Better Call Saul, though the show may have already referenced him. While the prequel/spinoff deals with the pre-Breaking Bad exploits of one Jimmy McGill (who later becomes Saul Goodman), fans are always on the hunt for connections between Saul and its parent program. Ever since it aired, arguably the biggest question has been if Saul's two most infamous clients, Walter White and Jesse, would make an appearance of any kind. Vince Gilligan is open to seeing those characters return, but it would have to be an organic fit.
That might make it difficult for Heisenberg to show up (he didn't know Saul until Breaking Bad), but there's still a chance Jesse could. Star Bob Odenkirk thinks Aaron Paul has to appear at some point, though it unfortunately won't happen in the forthcoming season four. However, Gilligan may have snuck in a subtle reference to Pinkman already.
During San Diego Comic-Con 2018, Screen Rant had the opportunity to attend a Better Call Saul press conference, where a possible role for Jesse was discussed. Michael Mando, who portrays Nacho on the show, offered his theory that suggests Pinkman was referenced in season 3:
"I don't know if this is true or not, but there could be a reference to Jesse Pinkman in Better Call Saul - if you pay attention. I don't know if it's Jesse Pinkman, but Krazy-8 mentions a new dealer who is brand new to his squad who is sort of not real good at it. And people think that's Jesse's first sort of introduction. I don't know if that's true or not."
As Mando says, this is simply his belief and shouldn't be considered canon at this point. It should be noted at the time these comments were made, Saul co-creators Gilligan and Peter Gould were not present, so they were unable to confirm or deny. That said, this isn't too outlandish a hypothesis. The first season of Breaking Bad established Jesse was partners with Emilio Koyama, Krazy-8's cousin. It would be reasonable to assume, then, that they were part of Krazy-8's operation for the Salamancas. This would be fittingly on-brand for Gilligan, who likes to be sly when it comes to this kind of stuff (see: the season 2 anagram "Fring's Back"). Instead of being overt and obvious with the Easter eggs, he prefers to provide clues for fans to figure out themselves, and it makes Better Call Saul a more rewarding viewing experience.
AMC recently renewed Better Call Saul for a fifth season, and with the titular character getting closer to completing his transformation into Saul Goodman, there's still a chance Jesse could appear. As fun as it would be to see Paul return, a case can be made it isn't needed. Saul has done an excellent job of emerging from Breaking Bad's shadow and stands on its own merits as an excellent television drama. If Jesse could help serve the story of Saul, then the creative team would be foolish to not include him, but they should never force him in for the sake of getting a reaction.
Better Call Saul season 4 premieres on AMC Monday, August 6, 2018.