First, it may be good to know that the MX-80 did not feature a bitmap mode for graphics, but only 64 symbols. The MX-80 also used 7 bit encoding, so no room for 8 bit graphic data. It was the MX-80 Type II that included it. So while the name is used simply as MX-80, it's always the MX-80 Type II we're talking.
Second, Just because some printer looked alike, doesn't mean it understood the commands equally well - or it's mechanic had the same quality.
So I assume now that these horizontal gaps are a result of the printer using only the top 8 of its 9 pins, but advancing the paper roll the height of the full 9 pins after every printed line.
Right... err ... at least basically.
If that was the reason, and if it was constant, it's most likely not the fault of the printer, but a wrong command to advance the paper (ESC+A).
But looking at the picture included, the gaps seam to be less than a pixel height. So it may not have been a wrong command, but simply less than desirable mechanics, always leaving a little gap. This is known from many printers, including genuine Epson MX-80. It doesn't make a big difference for text (especially if always about the same), but comes up in graphics.
These 'modulations' were simply part of bitmap graphics around 1980. Although, printers did advance (*1)
Why did the Epson MX-80 and its descendants not use the full 9 pins to print graphics?
Was there some technical reason why this was not possible or desirable?
Simply because it used 8 bit words (bytes) to for data transmission, transferring 9 bit data would have meant doubling the data transmission time amount for 12.5% faster printing. While this sounds strange from today - or even from the mid 80s, then the MX-80 was made, serial connections with less than 9600 bit/s were standard for printers.
Not to mention that it was way more convenient to handle graphic data in 8 bit chunks on computers operating with 8 bit bytes - this includes 'high level part' (*2) of the printer as well, especially for buffering
It wasn't until later when connections were by default faster (like in fast parallel interfaces) buffer sizes larger and print speed became a thing (*3). So printers like the 1986 Star LC-10 included a 9 pin mode.
With more memory comes more power :))
And were any of those printers smart enough to advance the paper roll by the height of 8 pins rather than 9 when printing graphics, thus avoiding those ugly gaps?
Already the original MX-80 (Type 1) could set the paper advancement in steps of 1/72 inch, essentially to each needle height. Its mechanics operated in 1/216 inch steps. The Type II inherited this. But here as well, advancement was less than perfect, leaving artifacts. They are even visible in the original Epson manual printing:
(Taken from Epson MX-80 Type II Manual p.83)
Long story short: Don't expect a 1965 VW-Beatle show features and performance of a luxury car (or in looking back the one of a 1990 VW Golf)
You could have bought a way more expensive printer, or waited 5-8 years for consumer products growing in quality ... or do like you (and we all) did, proudly show the results and live with these tiny, negligible artefacts.
*1 - It's like always, first generation is to fulfill the demand to print / compute / display / type / etc. at all, later generations will (and need to, to compete) improve and add 'luxury' features. The MX-80 is a great example, as it was the second versions that includes more than block graphics. And it took later printers to improve the mechanics to make bitmap look good - the first original one was only meant to make text.
*2 - It included two micro-controllers. An 8049 doing over all control as well as data transfer from/to interface, buffering, conversion from ASCII to bitmap and control of the needles and a (slave) 8041 handling the printing mechanic like head and paper advancement.
*3 - Like with *1, faster printing is a later improvement - with bitmap only way down the (time) line: printing -> printing bitmap -> printing bitmap without artifacts -> printing bitmap without and fast.
BTW, in other areas this sequence was different. For example the Siemens 9003, one of the fastest single head DMP in 1979. Here print quality was given up for speed. It's printouts didn't look great due simple fonts and inaccuracies of bidirectional printing, but it was dammed fast and all in a desktop case... well, desktop only in the sense it needs to be placed on a desk, so not being a stand alone device :))