Clarifications for the Curious:
-We may assume Jon is not still making his Chuck E. Cheese requests from yesterday, but making reservations at another restaurant for another date. The immediacy of placing the two strips next to each other might seem confusing, but Garfield tends to group running gags as close together as possible. This is not a technique utilized by many other strips, because it tends to draw attention to the format, and may indicate to the audience that the writer is idea-starved. Garfield, on the other hand, takes care to highlight its stock situations, which goes hand-in-hand with the strip's ongoing mission to provide variants on a narrow range of interests. This trait that does not go unappreciated by fans, as indicated by the nearly illiterate Wikipedia article (ugh) which attempts and fails to catalog these situations. The good news is that Jon is organizing another date, which means Liz wasn't put off by the robot mouse. Take that, Garfield!
-The "juice harp" Jon speaks of is chicken-speak neologism for Jew's harp. Jew's harp is not an antisemetic term, as far as etymologists know, though all the dictionaries I consulted (and followed by the presumed experts at the Jew's Harp Guild website) are unsure about the derivation. I can't fault Garfield for the editor-pleasing, nonconfrontational choice, but the uncommon terminology does confuse the gag a little. Why not just say "jaw harp", which is equally wrong, but more recognizable?
In any case, forget the poor romantic substitution of a Jew's harp for a violin, and Jon's dismay at how weird the world is: the real joke is that Jon turns to his cat to help him decide if he should accept the proposal. Garfield, either hoping to sabotage the date, or figuring it's going to be an evening of idiocy anyway, silently nudges Jon toward disaster. Leave it to Garfield to find a way to turn affirmation into a way of being negative.