The terrain of the Greco-Bulgarian border is completely different than the one in Dobruja. Mackensen attacked through flat terrain and the Danube. A simple wikipedia map can attest to that.
https://upload.wikimedia.org/wikipedia/commons/a/af/Romania-WW1-2.jpg. The invading Bulgarian army would have to attack via the river valleys of Vardar and Struma and a number of mountain ranges. The Struma Valley was protected by the Rupel fort. have in mind also, that they would be attacking an already entrenched Greek army in that terrain.
There are more noticeable differences and omissions as well:
a) The Greek army was a veteran army from the Balkan wars. It had the same experience as the Serbian one in August 1914. The Bulgarian army had been defeated by the Serbian-Greek Alliance in that war. In contrast, Romania's participation in the 2nd Balkan war involved only at best skirmishes - illness and not battle was the reason of Romanian casualties. Thus, in quality speaking, the Greek army of 1915 had experience in fighting the Bulgarians and high morale due to the recent (summer 1913) victory over them.
b) Between 1913 and 1915 there was a considerable expansion of the Greek Army. In fall 1915 15 divsions were mobilized in Greece and were sent to the northern border. The Greek army was mobilized right after the Bulgarian mobilization. So, by the end of the Serbian campaign the Greeks would have been fully mobilized and manning trenches in pre-determined defensive positions.
c) Strategically it would be a boon for the Entente. Entente would have gained not just the 15 aforementioned greek divisions but also: i) the considerable greek merchant fleet (9th or 10th in 1914 in the world), ii) the Greek Navy that was coveted by the RN that was streched to provide light units in the Easten Med. In the matter of war economy, the most valuable greek mineral of that time was magnesite for the steel industry.
d) Politically is a win for Greece because the National Schism is avoided with a October 1915 Bulgarian invasion.
e) At the short term, the Entente could land immediately in Thessaloniki 4 divisions to support the Greeks - the 10th British and the 57th, 122nd and 156th French as in OTL. The number of divisions would grow in the following months. It would be rather easy for the Entente to have by summer 1916 anything from 30-35 divisions in the Macedonian Front. Of course, the Greek participation would free Anglo-French manpower to be used in other fronts.
Overall, such a senario would constitute a clear net again for the Entente and would put more stress in the CP's manpower. The butterflies could affect the Romanian Front in 1916 with the vast majority of Bulgarian manpower tied in the Macedonian Front.